Keithley 6512 Service manual

Model 6512

Programmable Electrometer

Instruction Manual

Contains Operating and Servicing Information

W ARRANTY

Keithley Instruments, Inc. warrants this product to be free from defects in material and workmanship for a period of 1 year
from date of shipment.

Keithley Instruments, Inc. warrants the following items for 90 days from the date of shipment: probes, cables, rechargeable
batteries, diskettes, and documentation.

During the warranty period, we will, at our option, either repair or replace any product that proves to be defective.

To exercise this warranty, write or call your local Keithle y representative, or contact Keithle y headquarters in Cleveland, Ohio.
You will be given prompt assistance and return instructions. Send the product, transportation prepaid, to the indicated service
facility . Repairs will be made and the product returned, transportation prepaid. Repaired or replaced products are warranted for
the balance of the original warranty period, or at least 90 days.

LIMIT A TION OF W ARRANTY

This warranty does not apply to defects resulting from product modification without Keithley’s express written consent, or
misuse of any product or part. This warranty also does not apply to fuses, software, non-rechargeable batteries, damage from
battery leakage, or problems arising from normal wear or failure to follow instructions.

THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING ANY
IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. THE REMEDIES PRO­VIDED HEREIN ARE BUYER’S SOLE AND EXCLUSIVE REMEDIES.

NEITHER KEITHLEY INSTRUMENTS, INC. NOR ANY OF ITS EMPLOYEES SHALL BE LIABLE FOR ANY DIRECT,
INDIRECT, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OF ITS
INSTRUMENTS AND SOFTWARE EVEN IF KEITHLEY INSTRUMENTS, INC., HAS BEEN ADVISED IN ADVANCE
OF THE POSSIBILITY OF SUCH DAMAGES. SUCH EXCLUDED DAMAGES SHALL INCLUDE, BUT ARE NOT LIM­ITED TO: COSTS OF REMOVAL AND INSTALLATION, LOSSES SUSTAINED AS THE RESULT OF INJURY TO ANY
PERSON, OR DAMAGE TO PROPERTY.

Keithley Instruments, Inc. • 28775 Aurora Road • Cleveland, OH 44139 • 440-248-0400 • Fax: [phone] • http://www.keithle y.com

BELGIUM: Keithley Instruments B.V.
CHINA: Keithley Instruments China
FRANCE: Keithley Instruments Sarl B.P. 60 • 3, allée des Garays • 91122 Palaiseau Cédex • 01 64 53 20 20 • Fax: [phone]
GERMANY: Keithley Instruments GmbH Landsberger Strasse 65 • D-82110 Germering • 089/84 93 07-40 • Fax: [phone]
GREAT BRITAIN: Keithley Instruments Ltd
INDIA: Keithley Instruments GmbH Flat 2B, WILOCRISSA • 14, Rest House Crescent • Bangalore 560 001 • 91-80-509-1320/21 • Fax: [phone]
ITALY: Keithley Instruments s.r.l. Viale S. Gimignano, 38 • 20146 Milano • 02/48 30 30 08 • Fax: [phone]
NETHERLANDS: Keithley Instruments B.V. Postbus 559 • 4200 AN Gorinchem • 0183-635333 • Fax: [phone]
SWITZERLAND: Keithley Instruments SA Kriesbachstrasse 4 • 8600 Dübendorf • 01-821 94 44 • Fax: [phone]
TAIWAN: Keithley Instruments Taiwan 1 Fl. 85 Po Ai Street • Hsinchu, Taiwan, R.O.C. • 886-3572-9077• Fax: [phone]

Bergensesteenweg 709 • B-1600 Sint-Pieters-Leeuw • 02/363 00 40 • Fax: [phone]
Y uan Chen Xin Building, Room 705 • 12 Yumin Road, Dew ai, Madian • Beijing 100029 • 8610-62022886 • Fax: [phone]

The Minster • 58 Portman Road • Reading, Berkshire RG30 1EA • 0118-9 57 56 66 • Fax: [phone]

10/99

Model 6512 Programmable Electrometer

Instruction Manual

©1994, Keithley Instruments, Inc.

All rights reserved.

Cleveland, Ohio, U.S.A.

First Printing February 1994

Document Number: 6512-901-01 Rev. A

Manual Print History

The print history shown below lists the printing dates of all Revisions and Addenda created for this manual. The
Revision Level letter increases alphabetically as the manual undergoes subsequent updates. Addenda, which are
released between Revisions, contain important change information that the user should incorporate immediately
into the manual. Addenda are numbered sequentially. When a new Revision is created, all Addenda associated
with the previous Revision of the manual are incorporated into the new Revision of the manual. Each new Revi­sion includes a revised copy of this print history page.

Revision A (Document Number 6512-901-01)........................................................................... February 1994

All Keithley product names are trademarks or registered trademarks of Keithley Instruments, Inc.
Other brand and product names are trademarks or registered trademarks of their respective holders

Safety Precautions

The following safety precautions should be observed before using
this product and any associated instrumentation. Although some in­struments and accessories would normally be used with non-haz­ardous voltages, there are situations where hazardous conditions
may be present.

This product is intended for use by qualified personnel who recog­nize shock hazards and are familiar with the safety precautions re­quired to avoid possible injury. Read the operating information
carefully before using the product.

The types of product users are:

Responsible body is the individual or group responsible for the use

and maintenance of equipment, for ensuring that the equipment is
operated within its specifications and operating limits, and for en­suring that operators are adequately trained.

Operators use the product for its intended function. They must be

trained in electrical safety procedures and proper use of the instru­ment. They must be protected from electric shock and contact with
hazardous live circuits.

Maintenance personnel perform routine procedures on the product

to keep it operating, for example, setting the line voltage or replac­ing consumable materials. Maintenance procedures are described in
the manual. The procedures explicitly state if the operator may per­form them. Otherwise, they should be performed only by service
personnel.

Service personnel are trained to work on live circuits, and perform

safe installations and repairs of products. Only properly trained ser­vice personnel may perform installation and service procedures.

Users of this product must be protected from electric shock at all
times. The responsible body must ensure that users are prevented
access and/or insulated from every connection point. In some cases,
connections must be exposed to potential human contact. Product
users in these circumstances must be trained to protect themselves
from the risk of electric shock. If the circuit is capable of operating
at or above 1000 volts,

exposed.

As described in the International Electrotechnical Commission
(IEC) Standard IEC 664, digital multimeter measuring circuits
(e.g., Keithley Models 175A, 199, 2000, 2001, 2002, and 2010) are
Installation Category II. All other instruments’ signal terminals are
Installation Category I and must not be connected to mains.

Do not connect switching cards directly to unlimited power circuits.
They are intended to be used with impedance limited sources.
NEVER connect switching cards directly to AC mains. When con­necting sources to switching cards, install protective devices to lim­it fault current and voltage to the card.

Before operating an instrument, make sure the line cord is connect­ed to a properly grounded power receptacle. Inspect the connecting
cables, test leads, and jumpers for possible wear, cracks, or breaks
before each use.

For maximum safety, do not touch the product, test cables, or any
other instruments while power is applied to the circuit under test.
ALWAYS remove power from the entire test system and discharge
any capacitors before: connecting or disconnecting cables or jump­ers, installing or removing switching cards, or making internal
changes, such as installing or removing jumpers.

no conductive part of the circuit may be

Exercise extreme caution when a shock hazard is present. Lethal
voltage may be present on cable connector jacks or test fixtures. The
American National Standards Institute (ANSI) states that a shock
hazard exists when voltage levels greater than 30V RMS, 42.4V
peak, or 60VDC are present.

pect that hazardous voltage is present in any un­known circuit before measuring.

A good safety practice is to ex-

Do not touch any object that could provide a current path to the
common side of the circuit under test or power line (earth) ground.
Always make measurements with dry hands while standing on a
dry, insulated surface capable of withstanding the voltage being
measured.

The instrument and accessories must be used in accordance with its
specifications and operating instructions or the safety of the equip­ment may be impaired.

Do not exceed the maximum signal levels of the instruments and ac­cessories, as defined in the specifications and operating informa­tion, and as shown on the instrument or test fixture panels, or
switching card.

When fuses are used in a product, replace with same type and rating
for continued protection against fire hazard.

Chassis connections must only be used as shield connections for
measuring circuits, NOT as safety earth ground connections.

If you are using a test fixture, keep the lid closed while power is ap­plied to the device under test. Safe operation requires the use of a
lid interlock.

If a screw is present, connect it to safety earth ground using the
wire recommended in the user documentation.

!

The symbol on an instrument indicates that the user should re­fer to the operating instructions located in the manual.

The symbol on an instrument shows that it can source or mea­sure 1000 volts or more, including the combined effect of normal
and common mode voltages. Use standard safety precautions to
avoid personal contact with these voltages.

The

WARNING heading in a manual explains dangers that might

result in personal injury or death. Alw ays read the associated infor ­mation very carefully before performing the indicated procedure.

CAUTION heading in a manual explains hazards that could

The
damage the instrument. Such damage may invalidate the warranty.

Instrumentation and accessories shall not be connected to humans.

Before performing any maintenance, disconnect the line cord and
all test cables.

To maintain protection from electric shock and fire, replacement
components in mains circuits, including the power transformer, test
leads, and input jacks, must be purchased from Keithley Instru­ments. Standard fuses, with applicable national safety approvals,
may be used if the rating and type are the same. Other components
that are not safety related may be purchased from other suppliers as
long as they are equivalent to the original component. (Note that se­lected parts should be purchased only through Keithley Instruments
to maintain accuracy and functionality of the product.) If you are
unsure about the applicability of a replacement component, call a
Keithley Instruments office for information.

To clean an instrument, use a damp cloth or mild, water based
cleaner. Clean the exterior of the instrument only. Do not apply
cleaner directly to the instrument or allow liquids to enter or spill
on the instrument. Products that consist of a circuit board with no
case or chassis (e.g., data acquisition board for installation into a
computer) should never require cleaning if handled according to in­structions. If the board becomes contaminated and operation is af­fected, the board should be returned to the factory for proper
cleaning/servicing.

Rev.10/99

Table of Contents

1 General Information

1.1 Introduction..........................................................................................................................................................1-1

1.2 Features ...............................................................................................................................................................1-1

1.3 Warranty information ..........................................................................................................................................1-1

1.4 Manual addenda ..................................................................................................................................................1-2

1.5 Safety symbols and terms ...................................................................................................................................1-2

1.6 Specifications ......................................................................................................................................................1-2

1.7 Unpacking and inspection ...................................................................................................................................1-2

1.7.1 Shipment contents .....................................................................................................................................1-2

1.7.2 Instruction manual .....................................................................................................................................1-2

1.7.3 Repacking for shipment ............................................................................................................................1-2

1.8 Getting started .....................................................................................................................................................1-2

1.8.1 Preparation for use ....................................................................................................................................1-2

1.8.2 Quick start procedure ................................................................................................................................1-3

1.9 Accessories ..........................................................................................................................................................1-3

2 Operation

2.1 Introduction .........................................................................................................................................................2-1

2.2 Power-up procedure ............................................................................................................................................2-1

2.3 Power-up self-test and display messages ............................................................................................................2-2

2.3.1 RAM memory test .....................................................................................................................................2-2

2.3.2 Self-test and firmware revision level ........................................................................................................2-2

2.4 Front panel familiarization ..................................................................................................................................2-2

2.4.1 Controls .....................................................................................................................................................2-2

2.4.2 Display and indicators ...............................................................................................................................2-5

2.4.3 Tilt bail ......................................................................................................................................................2-5

2.5 Front panel programs ..........................................................................................................................................2-5

2.5.1 IEEE-488 address ......................................................................................................................................2-6

2.5.2 Exponent mode (alpha or numeric) ...........................................................................................................2-6

2.5.3 Calibration .................................................................................................................................................2-6

2.6 Rear panel familiarization ...................................................................................................................................2-6

2.7 Basic measurement techniques ...........................................................................................................................2-8

2.7.1 Warm-up period ........................................................................................................................................2-8

2.7.2 Input connections ......................................................................................................................................2-8

2.7.3 Making voltage measurements ..................................................................................................................2-9

2.7.4 Guarded operation ...................................................................................................................................2-11

2.7.5 Making current measurements ................................................................................................................2-12

2.7.6 Making charge measurements .................................................................................................................2-14

2.7.7 Resistance measurements ........................................................................................................................2-16

2.7.8 Using the ohms function as a current source ..........................................................................................2-17

i

2.8 Analog outputs .................................................................................................................................................. 2-18

2.8.1 2V analog output ..................................................................................................................................... 2-18

2.8.2 Preamp out .............................................................................................................................................. 2-19

2.9 Using external feedback ................................................................................................................................... 2-21

2.9.1 Electrometer input circuitry .................................................................................................................... 2-21

2.9.2 Shielded fixture construction .................................................................................................................. 2-22

2.9.3 External feedback procedure .................................................................................................................. 2-22

2.9.4 Non-standard coulombs ranges ............................................................................................................... 2-23

2.9.5 Logarithmic currents ............................................................................................................................... 2-23

2.9.6 Non-decade current gains ....................................................................................................................... 2-24

2.10 Using zero correct and baseline suppression .................................................................................................... 2-25

2.10.1 Zero correct and zero check .................................................................................................................... 2-25

2.10.2 Using suppression ................................................................................................................................... 2-25

2.11 Data storage ...................................................................................................................................................... 2-26

2.12 External triggering ............................................................................................................................................ 2-28

2.12.1 External trigger input .............................................................................................................................. 2-28

2.12.2 Meter complete output ............................................................................................................................ 2-29

2.12.3 Triggering example ................................................................................................................................. 2-29

2.13 Measurement considerations ............................................................................................................................ 2-30

2.13.1 Ground loops .......................................................................................................................................... 2-30

2.13.2 EIectrostatic interference ........................................................................................................................ 2-31

2.13.3 Thermal EMFs ........................................................................................................................................ 2-31

2.13.4 Electromagnetic interference (EMI)........................................................................................................ 2-31

2.13.5 Leakage resistance effects ...................................................................................................................... 2-32

2.13.6 Input capacitance effects ......................................................................................................................... 2-32

2.13.7 Source resistance .................................................................................................................................... 2-33

2.13.8 Source capacitance .................................................................................................................................. 2-34

2.14 Engineering units conversion ........................................................................................................................... 2-34

3 IEEE-488 Programming

3.1 Introduction ......................................................................................................................................................... 3-1

3.2 Device-dependent command programming ........................................................................................................ 3-1

3.2.1 Calibration value (A) ................................................................................................................................ 3-5

3.2.2 Reading mode (B) ..................................................................................................................................... 3-5

3.2.3 Zero check (C) .......................................................................................................................................... 3-6

3.2.4 Function (F) .............................................................................................................................................. 3-6

3.2.5 Data format (G) ......................................................................................................................................... 3-7

3.2.6 EOI and bus hold-off modes (K) .............................................................................................................. 3-8

3.2.7 Non-volatile memory storage (L) ............................................................................................................. 3-9

3.2.8 SRQ mask (M) and status byte format ..................................................................................................... 3-9

3.2.9 Baseline suppression (N) ........................................................................................................................ 3-12

3.2.10 Data store mode (Q) ................................................................................................................................ 3-13

3.2.11 Range (R) ................................................................................................................................................ 3-14

3.2.12 Trigger mode (T) .................................................................................................................................... 3-15

3.2.13 Status (U) ................................................................................................................................................ 3-16

3.2.14 Terminator (Y) ........................................................................................................................................3-20

3.2.15 Execute (X) ............................................................................................................................................. 3-20

3.2.16 Zero correct (Z) ....................................................................................................................................... 3-21

3.3 Bus connections ................................................................................................................................................ 3-22

3.4 Primary address ................................................................................................................................................ 3-22

3.5 Talk-only mode ................................................................................................................................................. 3-23

3.6 Front panel messages ........................................................................................................................................ 3-23

ii

3.6.1 Bus error ..................................................................................................................................................3-24

3.6.2 Number error ...........................................................................................................................................3-24

3.6.3 Trigger overrun error ...............................................................................................................................3-24

3.7 Bus data transmission times ..............................................................................................................................3-24

4 Applications

4.1 Introduction .........................................................................................................................................................4-1

4.2 Low-level leakage current measurements ...........................................................................................................4-1

4.3 Diode characterization ........................................................................................................................................4-3

4.4 Capacitor leakage measurements ........................................................................................................................4-4

4.5 Capacitance measurement ...................................................................................................................................4-5

4.6 Insulation resistance measurements ....................................................................................................................4-5

4.6.1 Unguarded resistance measurements ........................................................................................................4-5

4.6.2 Guarded resistance measurements ............................................................................................................4-6

4.6.3 V/I resistance measurements with external voltage source .......................................................................4-6

4.7 High-impedance voltmeter ..................................................................................................................................4-9

4.8 Voltage coefficients of high-megohm resistors ..................................................................................................4-9

4.9 Static charge detection ......................................................................................................................................4-11

5 Performance Verification

5.1 Introduction .........................................................................................................................................................5-1

5.2 Environmental conditions ...................................................................................................................................5-1

5.3 Initial conditions .................................................................................................................................................5-1

5.4 Recommended test equipment ............................................................................................................................5-1

5.5 Verification procedures .......................................................................................................................................5-2

5.5.1 Input current verification ..........................................................................................................................5-2

5.5.2 Amps verification ......................................................................................................................................5-2

5.5.3 Coulombs verification ...............................................................................................................................5-3

5.5.4 Volts verification .......................................................................................................................................5-3

5.5.5 Ohms verification ......................................................................................................................................5-4

5.5.6 Input impedance verification ....................................................................................................................5-6

6 Theory of Operation

6.1 Introduction .........................................................................................................................................................6-1

6.2 Overall functional description .............................................................................................................................6-1

6.3 Input preamplifier ............................................................................................................................................... 6-2

6.3.1 Input stage .................................................................................................................................................6-3

6.3.2 Gain stage ..................................................................................................................................................6-4

6.3.3 Output stage ..............................................................................................................................................6-4

6.3.4 Ohms voltage source .................................................................................................................................6-5

6.3.5 Zero check .................................................................................................................................................6-5

6.4 Additional signal conditioning ............................................................................................................................6-6

6.4.1 Ranging amplifier .....................................................................................................................................6-6

6.4.2 Multiplexer and buffer amplifier ...............................................................................................................6-6

6.4.3 -2V reference source .................................................................................................................................6-7

6.5 A/D converter ......................................................................................................................................................6-7

6.6 Digital circuitry ...................................................................................................................................................6-9

6.6.1 Microcomputer ..........................................................................................................................................6-9

iii

6.6.2 Memory elements ..................................................................................................................................... 6-9

6.6.3 Device selection ........................................................................................................................................ 6-9

6.6.4 IEEE-488 bus ............................................................................................................................................ 6-9

6.6.5 Input/output circuitry .............................................................................................................................. 6-10

6.6.6 Display circuitry ..................................................................................................................................... 6-10

6.7 Power supplies .................................................................................................................................................. 6-11

7 Maintenance

7.1 Introduction ......................................................................................................................................................... 7-1

7.2 Line voltage selection ......................................................................................................................................... 7-1

7.3 Line fuse replacement ......................................................................................................................................... 7-2

7.4 Calibration .......................................................................................................................................................... 7-2

7.4.1 Calibration cycle ....................................................................................................................................... 7-2

7.4.2 GUARD switch ......................................................................................................................................... 7-2

7.4.3 Calibration jumper .................................................................................................................................... 7-2

7.4.4 Required calibration equipment ................................................................................................................ 7-3

7.4.5 Environmental conditions ......................................................................................................................... 7-3

7.4.6 Calibration sequence ................................................................................................................................. 7-3

7.4.7 Input offset adjustment ............................................................................................................................. 7-3

7.4.8 Input current adjustment ........................................................................................................................... 7-4

7.4.9 Calibration program .................................................................................................................................. 7-4

7.4.10 Amps calibration ....................................................................................................................................... 7-4

7.4.11 Coulombs calibration ................................................................................................................................ 7-5

7.4.12 Volts calibration ........................................................................................................................................ 7-6

7.4.13 Ohms calibration ....................................................................................................................................... 7-7

7.4.14 Permanent storage of calibration constants .............................................................................................. 7-7

7.4.15 IEEE-488 bus digital calibration .............................................................................................................. 7-7

7.4.16 Additional calibration points .................................................................................................................... 7-8

7.5 Special handling of static-sensitive devices ....................................................................................................... 7-8

7.6 Disassembly instructions .................................................................................................................................... 7-9

7.7 Troubleshooting ................................................................................................................................................ 7-11

7.7.1 Recommended test equipment ................................................................................................................ 7-11

7.7.2 Power-up self-test ................................................................................................................................... 7-11

7.7.3 Self-diagnostic program .......................................................................................................................... 7-11

7.7.4 Power supply checks ............................................................................................................................... 7-13

7.7.5 Relay configuration ................................................................................................................................ 7-13

7.7.6 Ranging amplifier gain configuration ..................................................................................................... 7-14

7.7.7 A/D converter and display ...................................................................................................................... 7-14

7.7.8 Input and ranging amplifiers ................................................................................................................... 7-14

7.7.9 Digital circuitry ....................................................................................................................................... 7-14

7.7.10 Display board .......................................................................................................................................... 7-14

7.8 Input stage balancing procedure ....................................................................................................................... 7-17

7.9 Handling and cleaning precautions ................................................................................................................... 7-17

8 Replaceable Parts

8.1 Introduction ......................................................................................................................................................... 8-1

8.2 Parts list .............................................................................................................................................................. 8-1

8.3 Ordering information .......................................................................................................................................... 8-1

8.4 Factory service .................................................................................................................................................... 8-1

8.5 Component layout drawings and schematic diagrams ........................................................................................ 8-1

iv

Appendices

A IEE-488 Bus Overview.......................................................................................................................................A-1

B General Bus Commands...................................................................................................................................... B-1

C Interface Function Codes .................................................................................................................................... C-1

D Example Programs..............................................................................................................................................D-1

E Model 617/6512 Software Compatibility ............................................................................................................E-1

v

List of Illustrations

2 Operation

Figure 2-1 Model 6512 front panel ...............................................................................................................................2-3

Figure 2-2 Model 6512 rear panel ................................................................................................................................2-7

Figure 2-3 Input connector configuration .....................................................................................................................2-9

Figure 2-4 Connections for voltage measurements ....................................................................................................2-10

Figure 2-5 Meter loading considerations.....................................................................................................................2-10

Figure 2-6 Unguarded circuit.......................................................................................................................................2-11

Figure 2-7 Guarded circuit...........................................................................................................................................2-11

Figure 2-8 Guarded input connections ........................................................................................................................2-12

Figure 2-9 Current measurements................................................................................................................................2-13

Figure 2-10 Voltage burden considerations...................................................................................................................2-14

Figure 2-11 Coulombs connections...............................................................................................................................2-15

Figure 2-12 Resistance measurement connections........................................................................................................2-17

Figure 2-13 Typical 2V analog output connections.......................................................................................................2-19

Figure 2-14 Typical preamp out connections................................................................................................................2-20

Figure 2-15 Electrometer input circuitry (external feedback mode) .............................................................................2-21

Figure 2-16 Shielded fixture construction.....................................................................................................................2-22

Figure 2-17 “Transdiode” logarithmic current configuration........................................................................................2-24

Figure 2-18 Non-decade current gains...........................................................................................................................2-24

Figure 2-19 Equivalent input impedance with zero check enabled...............................................................................2-25

Figure 2-20 External trigger pulse specifications..........................................................................................................2-28

Figure 2-21 Meter complete pulse specifications..........................................................................................................2-29

Figure 2-22 External triggering example.......................................................................................................................2-30

Figure 2-23 Multiple ground points create a ground loop.............................................................................................2-30

Figure 2-24 Eliminating ground loops...........................................................................................................................2-30

Figure 2-25 Leakage resistance effects..........................................................................................................................2-32

Figure 2-26 Input capacitance effects............................................................................................................................2-32

Figure 2-27 Simplified model for source resistance and source capacitance................................................................2-33

3 IEEE-488 Programming

Figure 3-1 General data format......................................................................................................................................3-7

Figure 3-2 SRQ mask and status byte format..............................................................................................................3-10

Figure 3-3 U0 status word and default values .............................................................................................................3-17

Figure 3-4 U1 status (error condition) format..............................................................................................................3-18

Figure 3-5 U2 status (data condition) format...............................................................................................................3-19

Figure 3-6 IEEE-488 connector...................................................................................................................................3-22

Figure 3-7 IEEE-488 connections................................................................................................................................3-22

Figure 3-8 Model 6512 rear panel IEEE-488 connector..............................................................................................3-22

vii

4 Applications

Figure 4-1 Leakage current measurement..................................................................................................................... 4-2

Figure 4-2 Diode characterization................................................................................................................................. 4-3

Figure 4-3 Diode curves................................................................................................................................................ 4-4

Figure 4-4 Capacitor leakage tests ................................................................................................................................ 4-4

Figure 4-5 Capacitor measurement ............................................................................................................................... 4-5

Figure 4-6 Insulation resistance measurement (unguarded).......................................................................................... 4-6

Figure 4-7 Insulation resistance measurement (guarded).............................................................................................. 4-7

Figure 4-8 Insulation resistance measurement using external voltage source .............................................................. 4-8

Figure 4-9 Measuring high-impedance gate-source voltage......................................................................................... 4-9

Figure 4-10 Configuration for voltage coefficient studies............................................................................................ 4-10

Figure 4-11 Faraday cup construction........................................................................................................................... 4-11

5 Performance V erification

Figure 5-1 Connections for amps and coulombs verification ....................................................................................... 5-3

Figure 5-2 Connections for volts verification ............................................................................................................... 5-4

Figure 5-3 Connections for ohms verification............................................................................................................... 5-5

Figure 5-4 Connections for input impedance verification............................................................................................. 5-6

6 Theory of Operation

Figure 6-1 Overall block diagram ................................................................................................................................. 6-2

Figure 6-2 Basic configuration of electrometer preamplifier........................................................................................6-2

Figure 6-3 Electrometer preamplifier configuration ..................................................................................................... 6-3

Figure 6-4 Simplified schematic of input stage............................................................................................................. 6-4

Figure 6-5 Gain stage .................................................................................................................................................... 6-4

Figure 6-6 Output stage configuration in volts and ohms............................................................................................. 6-4

Figure 6-7 Output stage configuration in amps and coulombs...................................................................................... 6-5

Figure 6-8 Ohms voltage source simplified schematic ................................................................................................. 6-5

Figure 6-9 Zero check configuration in volts and ohms................................................................................................ 6-6

Figure 6-10 Zero check configuration in amps and coulombs........................................................................................ 6-6

Figure 6-11 Simplified schematic of ranging amplifier.................................................................................................. 6-6

Figure 6-12 Multiplexer and buffer................................................................................................................................. 6-7

Figure 6-13 Multiplexer phases....................................................................................................................................... 6-7

Figure 6-14 -2V reference source.................................................................................................................................... 6-7

Figure 6-15 A/D converter.............................................................................................................................................. 6-8

7 Maintenance

Figure 7-1 Calibration jumper location......................................................................................................................... 7-3

Figure 7-2 Input offset and current adjustment locations.............................................................................................. 7-4

Figure 7-3 Connections for Model 6512 calibration..................................................................................................... 7-5

Figure 7-4 Connections for external voltage source...................................................................................................... 7-6

Figure 7-5 Exploded view........................................................................................................................................... 7-10

A IEEE-488 Bus Overview

Figure A-1 IEEE-488 bus configuration ....................................................................................................................... A-1

Figure A-2 IEEE-488 handshake sequence................................................................................................................... A-3

Figure A-3 Command groups........................................................................................................................................ A-6

viii

List of Tables

2 Operation

Table 2-1 Front panel power-up default conditions .....................................................................................................2-1

Table 2-2 Display error messages ................................................................................................................................2-5

Table 2-3 Front panel program messages.....................................................................................................................2-5

Table 2-4 Typical display exponent values..................................................................................................................2-6

Table 2-5 Ohms function current output values.........................................................................................................2-18

Table 2-6 Typical 2V analog output values ............................................................................................................... 2-18

Table 2-7 Full-range PREAMP OUT values..............................................................................................................2-21

Table 2-8 Data store reading rates..............................................................................................................................2-27

Table 2-9 Voltage and percent error for various time constants ................................................................................2-33

Table 2-10 Minimum source resistance ....................................................................................................................... 2-33

Table 2-11 Equivalent voltage sensitivity of Model 6512 amps ranges.......................................................................2-34

Table 2-12 Engineering units conversion.....................................................................................................................2-34

3 IEEE-488 Programing

Table 3-1 Default conditions........................................................................................................................................3-2

Table 3-2 Device-dependent command summary........................................................................................................3-3

Table 3-3 Bus hold-off times........................................................................................................................................3-8

Table 3-4 Trigger to reading-ready times...................................................................................................................3-24

4 Applications

Table 4-1 Diode currents and voltages.........................................................................................................................4-3

5 Performance V erification

Table 5-1 Limits for amps verification.........................................................................................................................5-3

Table 5-2 Limits for volts verification .........................................................................................................................5-4

Table 5-3 Limits for ohms verification ........................................................................................................................ 5-5

6 Theory of Operation

Table 6-1 Memory mapping.........................................................................................................................................6-9

ix

7 Maintenance

Table 7-1 Line voltage selection (50-60Hz) ................................................................................................................ 7-1

Table 7-2 Line fuse selection....................................................................................................................................... 7-2

Table 7-3 Model 6512 amps calibration summary ...................................................................................................... 7-5

Table 7-4 Model 6512 volts calibration....................................................................................................................... 7-6

Table 7-5 Model 6512 ohms calibration...................................................................................................................... 7-7

Table 7-6 Recommended troubleshooting equipment ............................................................................................... 7-11

Table 7-7 Diagnostic program phases........................................................................................................................ 7-12

Table 7-8 Power supply checks.................................................................................................................................. 7-13

Table 7-9 Relay configuration ................................................................................................................................... 7-13

Table 7-10 Ranging amplifier gains............................................................................................................................. 7-14

Table 7-11 A/D converter checks ................................................................................................................................ 7-15

Table 7-12 Preamplifier checks ................................................................................................................................... 7-15

Table 7-13 Ranging amplifier checks.......................................................................................................................... 7-16

Table 7-14 Digital circuitry checks.............................................................................................................................. 7-16

Table 7-15 Display board checks................................................................................................................................. 7-16

Table 7-16 Input stage balancing................................................................................................................................. 7-17

A IEEE-488 Bus Overview

Table A-1 IEEE-488 bus command summary............................................................................................................. A-4

B General Bus Commands

Table B-1 General bus commands................................................................................................................................B-1

Table B-2 Default conditions........................................................................................................................................B-2

C Interface Function Codes

Table C-1 Model 6512 interface function codes ..........................................................................................................C-1

E Model 617/6512 Software Compatibility

Table E-1 Model 617 commands not used by Model 6512..........................................................................................E-1

x

1

General Information

1.1 Introduction

The Keithley Model 6512 Programmable Electrometer is a
highly sensitive instrument designed to measure voltage,
current, charge, and resistance. The measuring range of the
Model 6512 is between 10µV and 200V for voltage mea­surements, from 0.1fA and 20mA for current measurements,
between 0.1 Ω and 200G Ω for resistance measurements, and
in the range of 10fC and 20nC in the coulombs mode. Very
high input impedance and extremely low input offset current
allow accurate measurement in situations where many other
instruments would have detrimental ef fects on the circuit be­ing measured. A 4 ½ -digit display and standard IEEE-488 in­terface allow easy access to instrument data.

1.2 Features

Some important Model 6512 features include:

• Ideal for low-current measurements—Current resolu­tion of 0.1fA makes the Model 6512 ideal for very low­current measurements.

• Baseline Suppression—One-button suppression of a
baseline reading is available from the front panel or
over the IEEE-488 bus.

• One-shot Triggering—A front panel control for trigger ­ing one-shot readings from the front panel is included.

• Selectable Guarding—A selectable driven cable guard
is included to minimize the effects of leakage resistance
and stray capacitance.

• Standard IEEE-488 Interface—The IEEE-488 interface
allows full bus programmable operation of the Model

6512.

• Analog Outputs—Both preamp and 2V full-range ana­log outputs are included on the rear panel.

• 100-Point Data Store—An internal buffer that can store
up to 100 readings is accessible from either the front
panel or over the IEEE-488 bus.

• Minimum and maximum data points can be stored and
are accessible from the front panel or over the IEEE­488 bus.

•4 ½ -Digit Display—An easy-to-read front panel LED
display includes a 4 ½ -digit mantissa plus a two-digit
alpha or numeric exponent.

• Auto-ranging—Included for all functions and ranges.

• Digital Calibration—The instrument may be digitally
calibrated from the front panel or over the IEEE-488 bus.

• Zero Correct—A front panel zero correct control allows
you to cancel internal voltage offsets, optimizing accuracy .

1.3 W arranty information

W arranty information for your Model 6512 may be found in­side the front cover of this manual. Should you need to use
the warranty , contact your K eithley representati v e or the fac­tory for information on obtaining warranty service.

1-1

General Information

1.4 Manual addenda

Information concerning improvements or changes to the in­strument that occur after the printing of this manual will be
found on an addendum sheet included with this manual.
Please be sure that you read this information before attempt­ing to operate or service your instrument.

1.5 Safety symbols and terms

The following safety symbols and terms are used in this
manual and found on the instrument:

The ! symbol on the instrument indicates that you should
refer to the operating instructions in this manual for further
details.

The WARNING heading as used in this manual explains
dangers that might result in personal injury or death. Alw ays
read the associated information very carefully before per­forming the indicated procedure.

The

CAUTION heading used in this manual explains haz-

ards that could damage the instrument. Such damage may in­validate the warranty.

• Model 6512 Programmable Electrometer

• Model 237-ALG-2 Triax Cable

• Model 6512 Instruction Manual

• Additional accessories as ordered

1.7.2 Instruction manual

If an additional instruction manual is required, order the
manual package (Keithley Part Number 6512-901-00). The
manual package includes an instruction manual and all per­tinent addenda.

1.7.3 Repacking for shipment

Before shipping, the instrument should be carefully packed
in its original packing material or the equivalent.

If the instrument is to be returned to Keithley Instruments for
repair or calibration, include the following:

• Write ATTENTION REPAIR DEPARTMENT on the
shipping label.

• Include the warranty status of the instrument.

• Complete the service form at the back of this manual.

1.6 Specifications

Detailed Model 6512 specifications are located at the front of
this manual. Note that accuracy specifications assume that
the instrument has been properly zero corrected, as discussed
in Section 2.

1.7 Unpacking and inspection

The Model 6512 Programmable Electrometer was carefully
inspected before shipment. Upon receiving the instrument,
carefully unpack all items from the shipping carton and
check for any obvious signs of physical damage that might
have occurred during shipment. Report any damage to the
shipping agent at once. Retain the original packing material
in case shipment becomes necessary.

1.7.1 Shipment contents

The following items are included with every Model 6512
shipment:

1.8 Getting started

1.8.1 Preparation for use

Once the instrument is unpacked, it must be connected to an
appropriate power source as described below.

Line power

The Model 6512 is designed to operate from 105-125V or
210-250V power sources. (A factory configuration is avail­able for 90-110V and 195-235V ranges. Contact applications
department for details.) The factory set range is marked on
the rear panel of the instrument. Note that the line plug is de­signed to mate with the supplied 3-wire power cord.

CAUTION

Do not attempt to operate the instru­ment on a supply voltage outside the in­dicated range, or instrument damage
might occur.

1-2

General Information

Line voltage selection

The operating voltage of the instrument is internally select­able. Refer to Section 7 for the procedure to change or verify
the line voltage setting.

Line frequency

The Model 6512 may be operated from either 50 or 60Hz
power sources.

IEEE-488 primary address

If the Model 6512 is to be programmed over the IEEE-488
bus, it must be set to the correct primary address. The prima­ry address is set to 27 at the factory, but it may be pro­grammed from the front panel, as described in Section 3.

1.8.2 Quick start procedure

The Model 6512 Programmable Electrometer is a highly so­phisticated instrument with many capabilities. Although
there are a number of complex aspects about the instrument,
you can use the following basic procedure to get your instru­ment up and running quickly. F or more detailed information,
you should consult the appropriate section of the manual.
Complete, detailed operation concerning Model 6512 front
panel operation may be found in Section 2. If you wish to
control these functions over the IEEE-488 bus, consult Sec­tion 3.

1. Carefully unpack your instrument, as described in para­graph 1.7.

2. Locate the power cord, and plug it into the rear panel
power jack. Plug the other end of the line cord into an
appropriate power source that uses a grounded outlet.
See Section 2 for more complete information.

3. Connect a suitable triaxial cable to the rear panel INPUT
jack. (See paragraph 1.9 below for recommended triax­ial cables.) Make sure the rear panel V, Ω/ GUARD
switch is in the OFF position.

4. Press in on the front panel POWER switch to turn on the
power. The instrument will power up the auto-range
volts mode with zero check enabled.

5. Connect the input cable to the signal source to be mea­sured. Remember that the Model 6512 measures DC
voltages up to 200V.

6. Disable zero check to make a measurement.

7. Take the reading from the display.

8. To change to a different measuring function, simply
press the desired function button. For example, to mea­sure current, simply press the AMPS button.

1.9 Accessories

The following accessories are available for use with the
Model 6512.

INPUT cables

The triaxial cables listed below are recommended for mak­ing connections to the Model 6512 INPUT jack.

Model 237-ALG-2 Triax Cable—2m (6 ft.) of low-noise tri­ax cable (SC-22) terminated with a 3-slot male triax connec­tor on one end, and three alligator clips on the other end.
(This cable is supplied with the Model 6512.)

Model 7078-TRX-3 Triax Cable—A low-noise triax cable

0.9m (3 ft.) in length, terminated at both ends with 3-slot
male triax connectors. Also available in 3m (10 ft.) and 6m
(20 ft.) versions (Models 7078-TRX-10 and 7078-TRX-20
respectively).

SC-22 Triax Cable—Unterminated triax cable available in
custom lengths. Use with appropriate triax connector (such
as CS-631 described below) to construct complete cables.

IEEE-488 cables

Model 7007 IEEE-488 Cables—The Model 7007 cables are
shielded cables designed to connect the Model 6512 to the
IEEE-488 bus and are available in two similar versions. The
Model 7007-1 is 1m (3.3 ft.) in length, while the Model
7007-2 is 2m (6.6 ft.) long. Each cable is terminated with a
shielded IEEE-488 connector on each end, and each connec­tor has two metric screws.

Model 7008 IEEE-488 Cables—The Model 7008 cables are
similar IEEE-488 cables available in three lengths. The
Model 7003-3 is 0.9m (3 ft.) in length, while the Models
7008-6 and Model 7008-13 are 1.8m (6 ft.) and 4m (13 ft.)
in length respectively. Each cable is terminated with an
IEEE-488 connector on each end, and each connector has
two metric screws.

1-3

General Information

Trigger cables

The following cables are recommended for connecting the
Model 6512 METER COMPLETE OUTPUT and EXTER­NAL TRIGGER INPUT jacks to other instruments for e xter­nal triggering:

Model 7051-2 BNC Cable—A 0.6m (2 ft.) BNC to BNC ca­ble (RG-58C) with a 50 Ω characteristic impedance. Also
available in 1.5m (5 ft.) and 3.0m (10 ft.) lengths (Models
7051-5 and 7051-10 respectively).

Connectors and adapters

The following connectors and adapters are recommended for
use with the Model 6512:

• Model 237-TRX-T—3-slot male to dual female triax
tee adapter for use with Model 7078-TRX or other sim­ilar 3-slot triax cables.

• Model 6171—3-slot male to 2-lug female triax adapter.
Useful for connecting 2-slot triax cables to the Model
6512 INPUT jack.

Test fixtures

Models 6105 and 8008 Resistivity Chambers—The Models
6105 and 8008 are guarded test fixtures for measuring vol­ume and surface resistivities. The units assure good electro­static shielding and high insulation resistance. The complete
system requires the use of an external voltage source such as
the Model 230 as well as the Model 6512. Volume resistivity
up to 10

9

Ω -cm and surface resistivity up to 10

18

Ω can be

measured in accordance with ASTM test procedures. Sheet
samples 64 to 102mm (2 ½ × 4”) in diameter and up to

6.4mm ( ¼ ”) thickness can be accommodated. Excitation
voltages up to 1000V may be used.

Model 8006 Component Test Fixture—The Model 8006 is
specifically designed for making sensitive measurements on
standard package devices. Individual devices may be con­nected to one of eight device sockets, including axial, 4-, 8-,
10-, and 12-lead TO, and 28-pin DIPs. Instruments may be
connected using rear panel binding posts, BNC, or triax con­nectors.

Rack mount kits

• Model 7078-TRX-BNC—3-slot male triax to BNC
adapter. Allows connecting BNC cables to the Model
6512 INPUT jack.

• Model 7078-TRX-TBC—3-lug female triax bulkhead
connector with cap for assembly of custom panels and
interface connections.

• Model CAP-31—Protective cap/shield for the Model
6512 INPUT connector.

• Model CS-631—3-slot male triax cable mount connec­tor for use with SC-22 low-noise triax cable. Useful for
making custom cables for connections to the Model
6512 INPUT jack.

Model 1019 Rack Mounting Kits—The Model 1019A kits
are fixed or stationary rack mounting kits intended for
mounting instruments in standard 19-inch racks. The Model
1019A-1 mounts a single Model 6512 or other similar instru­ment, while the Model 1019A-2 mounts two Model 6512s or
similar instruments in a side-by-side configuration. The
Models 1019S-1 and 1019S-2 are similar rack mounting kits
with a sliding mount configuration.

1-4

2

Operation

2.1 Introduction

Operation of the Model 6512 may be divided into two gen­eral categories: front panel operation and IEEE-488 bus op­eration. This section contains information necessary to use
the instrument on a front-panel basis. Note that most of these
functions can also be programmed over the IEEE-488 bus, as
described in Section 3.

The following paragraphs contain a complete description of
Model 6512 front panel operation. First a complete descrip­tion of each front and rear panel function is presented. Next
the complete procedure for each of the measuring functions
is presented. Finally, the analog output and guard functions
are described along with a method to apply external feed­back.

2.2 Power-up procedure

Use the procedure below to connect the Model 6512 to line
power and power up the instrument.

1. Connect the female end of the power cord to the AC re­ceptacle on the rear panel of the instrument. Connect the
other end of the cord to a grounded AC outlet.

Failure to use a grounded outlet may re­sult in personal injury or death because
of electric shock.

CAUTION

Be sure that the power line voltage
agrees with the indicated range on the
rear panel of the instrument. Failure to
observe this precaution may result in in­strument damage. If necessary, the line
voltage may be changed, as described in
Section 7.

2. Turn on the power by pressing in the front panel POW­ER switch. The switch will be at the inner most position
when the instrument is turned on.

3. The instrument will power up in the volts function, in
the auto-range mode and with zero check enabled, as in­dicated by the associated front panel LEDs. All other
LEDs will be off when the instrument is first turned on.
T able 2-1 summarizes front panel power -up default con­ditions.

Table 2-1

Front panel power-up default conditions

WARNING

The Model 6512 is equipped with 3-wire
power cord that contains a separate
ground wire and is designed to be used
with grounded outlets. When proper
connections are made, instrument chas­sis is connected to power line ground.

Mode Power-up status

Function
Range
Zero Check
Suppression
Trigger
Data Store

Volts
Auto-range
Enabled
Disabled
Continuous, External
Disabled

2-1

Operation

2.3 Power-up self-test and
display messages

2.3.1 RAM memory test

RAM memory is automatically tested as part of the power­up procedure. If a RAM memory error occurs, the “rr” mes­sage will remain on the display. If the instrument was not
able to read the stored calibration constants and configura­tion, the decimal points in the two exponent digits will flash.

If such errors occur, the instrument may be partially or
completely inoperative. Refer to Section 7 for more com­plete details.

2.3.2 Self-test and firmware revision level

A power-up self-test may be run, and the firmware revision
level may be displayed by pressing and holding the TRIG
button when the unit is first turned on. During the test, all
front panel LEDs and the display segments will turn on as in
the example below:

ment), and problems develop, it should be
returned to Keithley Instruments for re­pair. See paragraph 1.7 for details on re­turning the instrument.

2.4 Front panel familiarization

The front panel layout of the Model 6512 is shown in Figure
2-1. The front panel may be divided into two sections: con­trols and display indicators. The following paragraphs de­scribe each of these items in detail.

2.4.1 Controls

All front panel controls except POWER are momentary con­tact switches. Many control buttons include an annunciator
light to indicate the selected mode. Some buttons have a sec­ondary function that is entered by pressing the SHIFT button
before pressing the desired button. All such buttons (except
ADJUST) are marked in yellow. The controls are color cod­ed into functional groups for ease of operation.

--1.8.8.8.8.*.*.

The instrument will then display the software revision level
when TRIG is released, for example:

A.1

(The revision level of your unit may be different.)

The instrument will then enter the diagnostic mode, which is
used as an aid in troubleshooting problems within the instru­ment. See Section 7 for details. Note that the power must be
turned off to remove the instrument from the diagnostic
mode.

NOTE

If the instrument is still under warranty
(less than one year from the date of ship-

1 POWER

The POWER switch controls AC power to the instrument.
Pressing and releasing the switch once turns the power on.
Pressing and releasing the switch a second time turns the
power off. The correct positions for on or off are marked on
the front panel immediately above the POWER switch.

2 SHIFT

The SHIFT button adds a secondary function to some of the
other front panel controls, including VOLTS, TRIG, RE­CALL, and PROGRAM SELECT. Note that the shift func­tion is entered by pressing SHIFT before the second button
rather than pressing the two simultaneously.

2-2

Operation

KEITHLEY

SHIFT

POWER

12 3

Figure 2-1

Model 6512 front panel

6

6512 PROGRAMMABLE ELECTROMETER

ELECTROMETER PROGRAM DATA STORE

VOLTS

OHMS

COUL

AMPS

RANGE

AUTO

MAX INPUT

250V

ZERO CHECK

ZERO CORRECT

SUPPRESS

TRIG SGL

7

METER
DATA

SELECT EXIT

ADJUST

4

8

STATUS

TALK
LISTEN
REMOTE

ON/OFF

RECALL EXIT

5

3 ELECTROMETER

The ELECTROMETER buttons control the measuring func­tions, selection of instrument ranges, and such items as zero
check, zero and suppression, and front panel triggering.

VOLTS—The VOLTS button places the instrument in
the DC volts measuring mode. When VOLTS is pressed,
the indicator next to the button turns on, showing that the
instrument is set for that mode. Note that the Model 6512
will be in this mode when it is first turned on. Pressing
SHIFT/VOLTS will place the instrument in the external
feedback mode, as described in paragraph 2.9.

OHMS—Pressing OHMS places the unit in the resis­tance measuring function. The indicator next to the
OHMS button will be illuminated when the instrument is
in this mode. Note that the instrument measures resis­tance using the constant-current method.

COUL—The Model 6512 may be set up to measure
charge by pressing the COUL button. The indicator next
to the COUL button will illuminate when the instrument
is set for this mode.

AMPS—Pressing AMPS switches the instrument to the
current-measuring function. The AMPS indicator will
turn on when the instrument is in this mode.

RANGE—These two buttons allow you to increment or
decrement the range the instrument uses. Pressing the ▲
button will move the instrument up one range each time
it is operated, while the ▼ button will move the instru­ment down range one increment each time it is pressed.
Note that pressing either of these buttons will cancel
auto-range if that mode was previously selected. The dis­play mantissa will remain blank until the first reading is
ready to be displayed.

AUTO—The AUTO button places the instrument in the
auto-range mode. While in this mode, the Model 6512
will switch to the best range to measure the applied sig­nal. Note that the instrument will be in the auto-range
mode when it is first turned on. Auto-ranging is a v ailable
for all functions and ranges. Auto-ranging may be can­celled either by pressing the AUTO button or one of the
two RANGE buttons.

2-3

Operation

ZERO CHECK—The zero check mode is used in con­junction with the ZERO CORRECT control to cancel
any offsets within the instrument and is also used as a
standby mode. Pressing ZERO CHECK once will enable
this mode, as shown by the associated indicator light.
When zero check is enabled, the electrometer input cir­cuit configuration changes (see paragraph 2.10). No
readings can be taken with zero check enabled. Pressing
ZERO CHECK a second time will disable this mode.
Zero check should be enabled when making connections
(except for coulombs) or when changing functions.

ZERO CORRECT—The zero correct mode works with
zero check to cancel electrometer offsets. If zero check is
enabled, pressing ZERO CORRECT will store a new of f­set value that will be used to cancel any offset. If the
range is changed while zero correct is enabled, the stored
value will be scaled accordingly. Zero correct may be
cancelled by pressing the ZERO CORRECT button a
second time. More information on using zero correct
may be found in paragraph 2.10.

SUPPRESS—The suppress mode allows you to cancel
external offsets or to store a baseline value to be subtract­ed from subsequent readings. For example, if you ap­plied 10V to the instrument and enabled suppress, that
value would then be subtracted from subsequent read­ings. Once suppress is enabled, the value is scaled when
the range is changed. Suppress may be disabled by press­ing the SUPPRESS button a second time and is cancelled
if the function is changed.

TRIG—The TRIG button allows you to enter the one­shot trigger mode and trigger single readings from the
front panel. To enter the one-shot mode, press SHIFT
then TRIG. The SGL indicator light will show that the in­strument is in the one-shot mode. Each time the TRIG
button is pressed, a single reading will be processed and
displayed. The displayed reading will flash when the
TRIG button is pressed. The one-shot trigger mode can
be cancelled by pressing SHIFT then TRIG a second
time. Additional information on triggering may be found
in paragraphs 2.12 and 3.2.12 in Sections 2 and 3.

4 PROGRAM

These keys allow access to Model 6512 front panel pro­grams, which control the IEEE-488 primary address, set al­pha or numeric exponent, and perform instrument calibra­tion. Front panel programs are described in paragraph 2.5.

SELECT/EXIT—This button enters the program mode
to allow access to parameters described above. Pressing
SELECT repeatedly causes the instrument to scroll
through a program menu. To cancel the program mode,
press SHIFT and then SELECT/EXIT in that order. Note
that the program mode is also cancelled by pressing SE­LECT/EXIT after a program parameter change is made.

ADJUST—These two buttons set program parameters,
as described in paragraph 2.5.

5 DATA STORE

The two DATA STORE buttons control the internal 100­reading data store mode of the instrument. Through these
two buttons, data storage may be enabled or disabled, the
storage rate may be selected, and readings may be recalled to
the front panel display. Paragraph 2.11 contains a complete
description of data store operation.

ON/OFF—This control enables or disables data store op­eration. In addition, reading rates can be selected by
holding the button in when first enabling data store.
When data store is enabled, the indicator light next to the
ON/OFF button will be on. Minimum and maximum val­ues are stored and up-dated as long as the ON/OFF LED
is on.

RECALL/EXIT—This single button serves to recall
readings previously stored by data store. Pressing and
holding this button causes the instrument to scroll
through data store locations as indicated on the display.
Once the desired reading number is displayed, releasing
the button causes the actual reading to be displayed. To
exit the recall mode, press SHIFT then EXIT.

2-4

Operation

2.4.2 Display and indicators

The operation of the 4 ½ digit display and various indicators
is described below. The display updates at about three read­ings per second in the continuous trigger mode.

6 Display

The Model 6512 has a display made up of a 4 ½ digit signed
mantissa as well as a two-digit signed exponent. The expo­nent can be represented either in scientific notation, or with
an alphanumeric subscript such as nA. The exponent display
mode can be changed with a front panel program, as de­scribed in paragraph 2.5. When scientific notation is used,
the decimal point remains fixed as in 1.9999, and the range
is indicated by the exponent. In addition, the display has a
number of front panel error messages that may occur during
operation; see Table 2-2.

Table 2-2

Display error messages

Message Description

OL Over-range input applied (- for nega-

tive value).

b Err Bus Error: Instrument programmed

while not in remote, or illegal com­mand or option sent.

n Err Number Error: Calibration value out of

limits.

t Err Trigger Error: Instrument triggered

while processing reading from previ­ous trigger.

8 STATUS indicators

These three indicators apply to operation of the Model 6512
over the IEEE-488 bus. The REMOTE indicator sho ws when
the instrument is in the IEEE-488 remote state, while the
TALK and LISTEN indicators show when the instrument is
in the talk and listen states respectively. See Section 3 for
more information on using the Model 6512 over the IEEE­488 bus.

2.4.3 Tilt bail

The tilt bail, which is located on the bottom of the instru­ment, allows the front panel to be elevated to a convenient
viewing height. To extend the bail, rotate it out 90° from the
bottom cover and latch it into place. To retract the bail, pull
out until it unlatches, and rotate it against the bottom cover.

2.5 Front panel programs

The Model 6512 has three front panel programs that can be
used to set the primary address, set the display exponent
mode (alpha or numeric), or calibrate the instrument from
the front panel. To select a program, press PROGRAM SE­LECT button repeatedly while observing the display . The in­strument will scroll through the available programs with
identifying messages, as shown in Table 2-3. When in the
program mode, the DATA STORE RECALL button is inop­erative; the data store mode may be turned off, but not on.
The operation of the various programs is described in the fol­lowing paragraphs. To exit a program, press SHIFT EXIT. If
a change was made, pressing SELECT alone will exit the
program.

7 METER and DATA indicators

The METER indicator identifies when the display is showing
a normal reading. The DATA LED indicates when the instru­ment is displaying DATA STORE information; a data store
reading is displayed when the DAT A LED is turned on. Usu­ally, the display will sho w normal readings (METER on), but
the RECALL button will switch the display to the data store
mode.

Table 2-3

Front panel program messages

Message Program description

IEEE Displays/sets IEEE-488 primary

address.
dISP Sets numeric or alpha exponent.
CAL* Allows instrument calibration.

* Not normally accessible unless enabled. See paragraph 7.4.9.

2-5

Operation

2.5.1 IEEE-488 address

Selection of the IEEE-488 address program is indicated by
the following message:

IEEE 27

Along with the message, the presently programmed IEEE­488 address (the factory default value of 27 in this example)
will be displayed. To select a new address, use the ADJUST
keys. When the desired value is shown in the display, press
SHIFT then SELECT EXIT to return to normal operation (or
if a change was made, simply press SELECT). For complete
information on using the Model 6512 over the IEEE-488 bus,
refer to Section 3.

2.5.2 Exponent mode (alpha or numeric)

The display exponent of the Model 6512 can be operated in
either the alpha mode or the numeric mode. In the alpha
mode, the exponent is given in actual units such as mA. In
the numeric mode, the exponent is given in scientific nota­tion. Table 2-4 gives typical examples, including units.

The display in the alpha mode appears as:

dISPm

Once the desired exponent mode is selected, press SHIFT
then SELECT EXIT to return to normal operation (or simply
PROGRAM SELECT if a change was made).

Table 2-4

Typical display exponent values

Engineering

Display

units

PA pA 10
nC nC 10
µA µA 10
mV mV 10
k Ω
M Ω
G Ω

k Ω
M Ω
G Ω

Scientific
notation Value

-12

A Picoamperes

-9

C Nanocoulombs

-6

A Microamperes

-3

V Millivolts

3

10
10
10

Ω

6

Ω

9

Ω

Kilo-ohms
Mega-ohms
Giga-ohms

To select the exponent program, scroll through the program
menu until the following message is displayed:

dISP

Use either of the ADJUST buttons to set the exponent to the
desired mode. In the numeric mode, the display might show:

dISP -3

2.5.3 Calibration

An advanced feature of the Model 6512 is its digital calibra­tion program. The instrument can be calibrated from the
front panel or over the IEEE-488 bus. To use the front panel
calibration program, refer to the calibration procedures out­lined in Section 7, paragraph 7.4.9.

2.6 Rear panel familiarization

The rear panel of the Model 6512 is shown in Figure 2-2.

2-6

PEAK

500V

OFF

OFF

V, Ω GUARD

LO

250V PEAK

V, Ω GUARD

INPUT

INPUT

TRIAX

PREAMP

ON

GUARD
(FOLLOWS

INPUT)

DO NOT FLOAT INPUT LO
WITH PREAMP OUT COM
CONNECTED TO EARTH.

!

ON

10K

!

100

CAUTION:

ADDRESS ENTERED WITH

FRONT PANEL PROGRAM

6

PREAMP OUT

2V ANALOG OUTPUT

COM

!

IEEE 488 INTERFACE

SH1

AH1T5TE0L4LE0

SR1

RL0

PP0

PREAMP OUT

!

ANALOG OUTPUT

DC1C0E1

7

COM

2V

SLOWBLOW

1/8A 90-125V

1/16A 180-250V

LINE

FUSE

9

8

COMPLETE

10 11

IEEE COMMON

LINE VOLTAGE

SELECTED

(INTERNAL)

105-125V
210-250V

LINE RATING

50-60 HZ

15VA MAX

EXTERNAL

TRIGGER

INPUT

METER

OUTPUT

Operation

1

23

Figure 2-2

Model 6512 rear panel

1 INPUT

The INPUT connector is a 3-lug female triax connector to be
used for all electrometer signal inputs. Note that you should
not confuse a triaxial connector with the BNC type that is
used for the EXTERNAL TRIGGER and ELECTROME­TER COMPLETE connections. Also, do not attempt to force
2-lug triaxial connector onto the INPUT connector. (See
paragraph 1.9 for details on 2-lug to 3-slot triax adapters.)

CAUTION

Do not float INPUT LO with preamp
out COM connected to earth (chassis
ground).

2 V, Ω GUARD Switch

4

5

3 IEEE-488 Connector

This connector is used to connect the instrument to the IEEE­488 bus. IEEE-488 function codes are marked above the con­nector.

4 LINE FUSE

The LINE FUSE, which is accessible on the rear panel, pro­vides protection for the AC power line output. For informa­tion on replacing this fuse, refer to Section 7.

5 AC Receptacle

Power is applied through the supplied power cord to the AC
receptacle. Note that the supply voltage is marked adjacent
to the receptacle.

The V, Ω GUARD switch adds capabilities for connecting a
guard voltage to the inner shield of the input cable. Guarding
is useful in the volts and ohms modes to speed up response
time and minimize the effects of leakage resistance and stray
capacitance. Note that guarded operation is not recommend­ed in amps or coulombs modes. See paragraph 2.7.4 for more
information on guarded operation.

6 Chassis Ground

This jack is a 5-way binding post that is connected to instru­ment chassis ground. It is intended for use in situations re­quiring an accessible chassis ground terminal. A shorting
link is supplied and connected between the chassis ground
and COM terminals.

2-7

Operation

7 PREAMP OUT

The PREAMP OUT jack provides a guard output for voltage
and resistance measurements. This output can also be used as
an inverting output or with external feedback when measur­ing current or charge. The PREAMP OUT has a maximum
output value of ±300V and uses a standard 5-way binding
post.

WARNING

Hazardous voltage may be present at
the PREAMP OUT, depending on the
input signal.

8 COM Terminal

The COM terminal is a 5-way binding post that provides a
low connection for both the 2V AN ALOG OUTPUT and the
PREAMP OUT . This terminal is also used for input lo w con­nection when in guarded mode; COM is internally connected
to input low through a 100 Ω

resistor.

CAUTION

2.7 Basic measurement techniques

The paragraphs below describe the basic procedures for us­ing the Model 6512 to make voltage, resistance, charge, and
current measurements.

2.7.1 Warm-up period

The Model 6512 is usable immediately when it is first turned
on. However, the instrument must be allowed to warm up for
at least two hours to achieve rated accuracy.

NOTE

While rated accuracy is achieved after the
two-hour warm up period, input bias cur­rent may require additional time to be re­duced to its optimum level. Allow two
more hours for input bias current to settle
to less than 10fA and eight hours for set­tling to less than 5fA. In sensitive applica­tions, it is preferable for the unit to be left
on continuously.

Do not connect PREAMP OUT, COM,
or 2V ANALOG OUTPUT to earth
ground when floating the input.

9 2V ANALOG OUTPUT

The 2V ANALOG OUTPUT provides a scaled 0-2V output
from the electrometer (2V output for full range input). The
output uses a standard 5-way binding post and is inverting in
the volts and ohms modes.

10 METER COMPLETE OUTPUT

This BNC connector provides an output pulse when the
Model 6512 has completed a reading; it is useful for trigger­ing other instrumentation.

11 EXTERNAL TRIGGER INPUT

This BNC connector can be used to apply external trigger
pulses to the Model 6512 to trigger the instrument to take one
or more readings, depending on the selected trigger mode.

2.7.2 Input connections

The rear panel INPUT connector is a Teflon-insulated, 3-lug
female triax connector intended for all input signals to the
Model 6512. As shown in Figure 2-3, the center terminal is
high, the inner ring or shield is low, and the outer shield is
connected to instrument chassis ground. In the guarded
mode, the inner shield is driven at guard potential, while the
outer shield is chassis ground.

NOTE

The INPUT connector must be kept clean to
maintain high input impedance. Place the
supplied rubber dust cap on the INPUT con­nector when the instrument is not in use.

2-8

INPUT HI

INPUT HI

60Hz sine wave (10 seconds maximum
in mA ranges). Exceeding this value
may cause damage to the instrument.

Operation

INPUT LO

CHASSIS

GROUND

A. UNGUARDED

(V, Ω GUARD OFF)

GUARD

CHASSIS

GROUND

100Ω

COM

B. GUARDED

(V, Ω GUARD ON)

Figure 2-3

Input connector configuration

The supplied Model 237-ALG-2 cable is designed to mate with
the INPUT connector. The other end of the Model 237-ALG-2
is terminated with three alligator clips. Input high is color coded
in red, input low is colored black, and chassis ground is color
coded in green. Keep in mind that these connections are for the
unguarded mode. In the guarded mode, red is high, black is
guard, and green is chassis ground. The COM binding post pro­vides a connection to input low through 100 Ω for use in the
guarded mode.

NOTE

It is recommended that zero check be en­abled when connecting or disconnecting
input signals.

2.7.3 Making voltage measurements

The Model 6512 can be used to measure voltages in the
range of ±10µV to ±200V. In principle, the instrument oper­ates much like an ordinary DMM, but its special characteris­tics allow it to make measurements in cases where an
ordinary DMM would be unable to perform well. In particu­lar, the very high input resistance of 200T Ω (2 × 10
lows it to accurately measure voltage sources with high
internal resistances. In contrast, an ordinary DMM may have
an input resistance of only 10M Ω , resulting in inaccurate
measurements because of instrument loading.

Use the following procedure to make voltage measurements:

1. Turn on instrument po wer, and allo w the unit to warm up
for two hours to reach rated accuracy.

2. Check to see that the voltage function is selected by
pressing the VOLTS button. Use the auto-range mode,
or select the desired range with the ranging pushbuttons.

3. To achieve specified accuracy, especially on the lower
ranges, it is recommended that you zero the instrument.
To do so, first enable zero check, and then press the
ZERO CORRECT button. Correcting zero on the lo west
range of any function will correct all ranges because of
internal scaling.

14

Ω ) al-

WARNING

The maximum common-mode input volt­age (the voltage between input low and
chassis ground) is 500V peak. Exceeding
this value may create a shock hazard.

CAUTION

Connecting PREAMP OUT, COM, or 2V
ANALOG OUTPUT to earth while float­ing the input may damage the instrument.

CAUTION

The maximum voltage between input
high and input low is 250V RMS, DC to

NOTE

The input circuit configuration changes
with zero check enabled. See paragraph

2.10.1 for details.

4. Connect the supplied Model 237-ALG-2 triaxial input
cable or other suitable triax cable to the rear panel IN­PUT jack. For sources with high output resistance, the
cable should be kept as short as possible to minimize ca­ble capacitance.

5. If response time and leakage resistance are consider­ations, place the instrument in the guarded mode as de­scribed in paragraph 2.7.4.

6. Connect the other end of the cable to the voltage to be
measured, as shown in Figure 2-4. Disable zero check.

2-9

Operation

A

B

7. Take the reading directly from the display. The exponent can be placed either in the alpha or numeric mode, as described
in paragraph 2.5.

Voltage measurement considerations

Two factors are of concern when making voltage measurements, especially for voltage sources with high output (source) resis­tances. For one thing, the loading effects of the measuring instrument come into play at the high resistance lev els in v olved. Sec­ondly, the distributed capacitance of the source, the input cable, and the input circuit of the instrument itself are a factor when
making these measurements.

Figure 2-5 demonstrates how meter loading can affect measurement accuracy. Here, a voltage source with a value E
source resistance R

is connected to the input of the electrometer. The input resistance of the electrometer is R

S

. The percent

IN

error due to loading can be calculated as follows:

100R

S

ERROR

%

------------------------=

RSR

+

IN

Thus, to keep the error under 0.1%, the input resistance must be about 1000 times the value of the source resistance, R
that the Model 6512 input resistance is ≥ 200G Ω , but the cable resistance appears in parallel.

Figure 2-5

Meter loading considerations

INPUT

TRIAX CABLE

SOURCE

E

R

S

S

R

IN

METER

V

and a

S

. Note

S

ON

OFF

V, Ω GUARD

MODEL 6512

V

S

GND

Figure 2-4

Connections for voltage measurements

2-10

HI

LO

INPUT AMPLIFIER

PREAMP OUT

COM

At very high resistance levels, the very large time constants
created by even a minimal amount of capacitance can slow
down response time considerably. For example, measuring a
source with an internal resistance of 100GΩ would result in
an RC time constant of one second when measured through
a cable with a nominal capacitance of 10pF. If 1% accuracy
is required, a single measurement would require at least five
seconds. Note that typical input cables have unguarded ca­pacitance (high to inner shield) of 120 to 150pF per meter.

Basically, there are two ways to minimize this problem: (1)
keep the input cable as short as possible, and (2) use guarding.
With the first method, there is a limit as to how short the cable
can be. Using guarding can reduce these effects and decrease
settling times by up to a factor of 1,000. The Model 6512 rear

Operation

O

6512
PREAMP

Z

L

R

S

E

S

A=1

OV

Z

S

(

SHIELD)

E

O

panel V, Ω GUARD switch allows guarding to be easily ap­plied to the input circuit; see paragraph 2.7.4 for details.

At low signal levels, noise may affect accuracy. Shielding of
the unknown voltage can reduce noise effects substantially.
When using shielding, the shield should normally be con­nected to input low , although connecting the shield to chassis
ground may yield better results in some cases.

2.7.4 Guarded operation

Guarding consists of using a conductor driven by a low-im­pedance source to totally surround the leads carrying a high­impedance signal. The output of this low-impedance source
is kept at the same potential as the signal itself, resulting in
drastically reduced leakage currents.

To approach the concept of guarding, let us first review the
unguarded circuit shown in Figure 2-6. The measured signal
is represented by the voltage source ES and the source resis­tance RS. Cable leakage impedance is represented by ZL.
The source resistance and leakage impedance form a voltage
divider that attenuates the source voltage as follows:

ZLE

S

---------------------=

E

O

ZLR

+

S

ES ZL

EO =

RS + ZL

R

Z

L

R

S

E

S

A. BASIC CONFIGURATION

E

O

S

E

S

B. EQUIVALENT CIRCUIT

Z

L

ty-gain amplifier with a high input impedance and low output
impedance is used. The input of the amplifier is connected to
the signal, while the output is used to drive the shield. Since
the amplifier has unity gain, the potential across ZL is typi­cally <1mV, so no leakage current flows. Leakage between
the cable shield and ground may be considerable, but it is of
no consequence since that current is supplied by the low-im­pedance source, rather than by the signal itself.

Figure 2-7

Guarded circuit

When the rear panel V, Ω/ GUARD switch is placed in the
ON position, guard potential is placed on the inner shield of
the triaxial cable. The outer shield remains at chassis ground.
Thus, it is necessary to use the COM terminal for low signal
connections, as shown in Figure 2-8. For very critical mea­surements, a shielded, guarded enclosure should be used.

WARNING

Hazardous voltage (up to 300V) may be
present on the inner shield when V, Ω/

E

GUARD is on, depending on the input
signal. A safety shield, connected to chas­sis ground is recommended when mak­ing voltage measurements over 30V or
for guarded resistance measurements.

Figure 2-6

Unguarded circuit

The use of guarding is not recommended

NOTE

for the amps or coulombs functions.

Thus, to keep the error due to leakage resistance under 0.1%,
the leakage resistance must be at least 1,000 times the source
resistance value.

Guarding the circuit minimizes these effects by driving the
shield at signal potential, as shown in Figure 2-7. Here, a uni-

The PREAMP OUT terminal may be used for guarding in
the volts and ohms modes in a similar manner. In this mode,

2-11

Operation

TRIAX CABLE

OFF

V, Ω GUARD

HI

LO

SAFETY SHIELD

Figure 2-8

Guarded input connections

INPUT

ON

MODEL 6512

HI

GUARD

GND

COM

PREAMP OUT

LO

COM

A. CONNECTIONS

V, Ω
GUARD
SWITCH

INPUT AMPLIFIER

100Ω

S

B. EQUIVALENT CIRCUIT (VOLTS MODE SHOWN)

HI

GUARD

GND

LO

WARNING: USE SAFETY SHIELD FOR SIGNALS

RANGING AMPLIFIER

S

HI

MEASURED

DEVICE

LO

GUARD

SAFETY SHIELD

ABOVE 30V (VOLTS) AND OHMS

TO A/D

CONVERTER

2V ANALOG

OUTPUT

the preamplifier acts as a unity-gain amplifier with low out­put impedance.

WARNING

Hazardous voltage (up to 300V) may be
present at the PREAMP OUT terminal,
depending on the input signal.

2.7.5 Making current measurements

The Model 6512 can resolve currents as low as 0.1fA

-16

(10

A), and measure as high as 20mA in 11 ranges. The
Model 6512 exhibits low input voltage b urden and extremely
low input offset current. The low voltage burden is achieved
because the Model 6512 measures current as a feedback type
picoammeter, rather than the shunt method used by many
DMMs.

CAUTION

Safe operation and good measurement
practice dictates the use of an external
resistor when necessary to limit currents
to less than 30mA.

NOTE

After measuring high voltage in volts, or
following an overload condition in ohms,
it may take a number of minutes for input
current to drop to within specified limits.
Input current can be verified by placing a
shielded cap on the INPUT jack and then
connecting a jumper between the COM
and chassis ground terminals. W ith the in­strument on the 2pA range and zero check
disabled, allow the reading to settle until
the instrument is within specifications.

2-12

Operation

To measure current with the Model 6512, use the following
procedure:

1. Turn on the po wer , and allo w the instrument to warm up
for at least two hours to obtain rated accuracy.

2. Select the current mode by pressing the AMPS b utton on
the front panel. Set the V, Ω/ GUARD switch to OFF.

3. T o achiev e rated accuracy , select the 2pA range, zero the
instrument by enabling zero check and then pressing the
ZERO CORRECT button. After zero correction, select
the desired range, or use auto-ranging if desired.

TRIAX CABLE

INPUT

OFF

ON

VΩ, GUARD

MODEL 6512

INPUT

AMPLIFIER

Z

INPUT

HI

I

S

LO

S

GND

F

PREAMP

OUT

COM

4. Connect a suitable triax cable to the rear panel INPUT
jack. Connect the other end of the circuit to be measured
as shown in Figure 2-9. Shielding will be required for
low-level measurements. Connect the shield to input
low.

5. Disable zero check, and allow the reading to settle.

6. Read the current value directly from the display . The e x­ponent may be placed either in the alpha or numeric
modes, as described in paragraph 2.5.

HI

I

S

MEASURED

CURRENT

SHIELD

BELOW 1µA

TO A/D

CONVERTER

2V ANALOG

OUTPUT

A. CONNECTIONS

RANGING
AMPLIFIER

B. EQUIVALENT CIRCUIT

LO

100Ω

RECOMMENDED

S

Figure 2-9

Current measurements

2-13

Operation

Current measurement considerations

At very low lev els (in the picoampere range or below), noise
currents generated in the cable or from other sources can af­fect measurements. Currents generated by triboelectric ef­fects are a primary cause of noise currents generated in
connecting cables. These currents are generated by charges
created at the junction between a conductor and an insulator
because of friction. Coaxial and triaxial cables are especially
prone to such noise currents, which are generated by cable
flexing. To minimize these effects, the cable should be tied
down firmly to minimize any flexing. Also, special lo w-noise
cable, constructed with graphite between the shield and insu­lator, is available to minimize these effects (see paragraph

1.9). However, even with low-noise cables, several tens of
femtoamps of noise currents can be generated by cable
movement.

SOURCE

R

E

S

I

S

METER

E

IN

(VOLTAGE

BURDEN)

Figure 2-10

Voltage burden considerations

2.7.6 Making charge measurements

I =

ES - E

IN

R

S

Voltage burden is frequently a consideration when making
current measurements. Ideally, the input voltage burden
should be zero for the instrument to have absolutely no effect
on the circuit it is measuring. If the voltage burden is too high,
its effects can degrade measurement accuracy considerably.

T o see ho w voltage b urden can upset measurement accurac y,
refer to Figure 2-10. A source, represented by E

with an out-

S

put resistance RS, is shown connected to the input of a pi­coammeter. The voltage burden is represented by a constant
voltage source at the input as EIN. If EIN were zero, the cur­rent as seen by the meter would simply be:

E

S

I

-------=

R

S

However, if EIN has a non-zero value, the current now be­comes:

ESE

–

I

IN

-----------------------=

R

S

Note that the Model 6512 voltage burden is typically 1mV or
less. Additional considerations include source resistance and
capacitance, as discussed in paragraph 2.13.

The Model 6512 is equipped with three coulombs ranges to
resolve charges as low as 10fC (10

-14

C) and measure as high

as 20nC (20 × 10-9C). When the instrument is placed in one
of the coulombs ranges, an accurately known capacitor is
placed in the feedback loop of the amplifier so that the volt­age developed is proportional to the integral of the input cur ­rent in accordance with the formula:

i dt

Q

S

------- -==

C

1

----

V

idsst

∫

∫

C

The voltage is scaled and displayed as charge.

NOTE

After measuring high voltages in volts, or
following an overload condition in ohms,
it may take a number of minutes for the in­put current to drop to within specified lim­its. Input current can be verified by placing
a shielded cap on the INPUT jack and then
connecting a jumper between the COM
and chassis ground terminals. W ith the in­strument on the 2pA range and zero check
disabled, allow the reading to settle until
the instrument is within specifications.

2-14

Operation

Normal charge measurements

Use the following procedure to measure charge with the
Model 6512:

1. Turn on the power, and allow a two-hour warm up peri­od for rated accuracy.

2. Place the instrument in the coulombs mode by pressing
the COUL button. Set V, Ω/ GUARD to OFF.

3. To achieve rated accuracy, place the instrument on the
200pC range, and zero the instrument by enabling zero
check and then pressing the ZERO CORRECT button.

4. Select the desired range, or use auto-ranging, if desired.

5. Disable zero check. A small amount of zero check hop
(sudden change in the reading) may be observed when

TRIAX CABLE

INPUT

zero check is disabled. If desired, enable suppress to null
out any zero check hop, which typically will be in the
10-25 count range.

6. Connect the triax cable to the INPUT jack. Connect the
other end of the cable to the circuit being measured, as
shown in Figure 2-11. For low-level measurements,
shielding may be required.

NOTE

Do not connect the circuit to the instru­ment with zero check enabled.

7. Read the charge value from the display. The exponent
may be placed either in the alpha or numeric modes as
described in paragraph 2.5.

NOTE: LEAVE QS DISCONNECTED

UNTIL ZERO CHECK DISABLED

HI

Q

S

MEASURED

CHARGE

LO

Q

Figure 2-11

Coulombs connections

ON

OFF

V, Ω GUARD

S

MODEL 6512

HI
LO

GND

INPUT AMPLIFIER

C

F

1000pF

S

PREAMP

OUT

COM

A. CONNECTIONS

B. EQUIVALENT CIRCUIT

S

100Ω

SHIELD (OPTIONAL)

S

TO A/D

CONVERTER

2V ANALOG

OUTPUT

2-15

Operation

Using the coulombs function to measure current

Note that the coulombs function can also be used to measure
current. The advantage of doing so is that noise in the mea­surement is substantially reduced because of the integrating
process. To measure current using the coulombs function,
proceed as follows:

1. Place the instrument in the coulombs function, and se­lect the desired range, or use auto-ranging, if desired.

2. Enable zero check, and connect the current to be mea­sured to the INPUT jack (see Figure 2-9).

3. Disable zero check, and note the charge measurement at
the end of a specific interval of time (for example, 10
seconds).

4. To determine the current, simply divide the measured
charge by the time in seconds. For example, if a charge
of 12nC is seen after a 10-second interval, the current is
12nC/10 = 1.2nA. (Using Data Store at a 10-second rate
can simplify the process.)

5. As an alternative to the abov e procedure, connect a chart
recorder to the 2V AN ALOG OUTPUT (paragraph 2.9),
and graph the measured charge. Since the current is giv­en by I=dQ/dt, the current at any point is equal to the
slope of the graph at that point, after applying the appro­priate scaling factor (100pC/V, 200pC range; 1nC/V,
2nC range; 10nC/V, 20nC range).

CAUTION

Connecting PREAMP OUT, COM, or
2V ANALOG OUTPUT to earth while
floating input may damage the instru­ment.

Charge measurement considerations

A primary consideration when making change measure­ments is the input offset current of the integrating amplifier.
Any such current is integrated along with the input signal
and reflected in the final reading. The Model 6512 has a max-

imum input offset current of 5 × 10

-15

A at 23°C. This value

doubles every 10°C. This input offset current translates into
a charge of 5 × 10

-15

C per second at a temperature of 23°C.
This value must be subtracted from the final reading to ob­tain the correct value.

When using an external voltage source, the input current
should be limited to less than 1mA by placing a resistor in se­ries with the high input lead. The value of this resistor should
be at least: R=1000 × VS (in ohms) where V is the voltage
source value, or the compliance of the current being integrated.

2.7.7 Resistance measurements

The Model 6512 makes resistance measurements using the
constant-current method. (A current is forced through the
DUT, and the voltage across the DUT is measured.). The in­strument can resolve resistances as low as 0.1Ω and measure
as high as 200GΩ.

To measure resistance with the Model 6512, use the follow­ing procedure:

1. Turn on the po wer, and allow a two-hour warm-up peri­od for rated accuracy.

2. Press the OHMS button to select the ohms mode.

3. For maximum accuracy, place the instrument on the
2kΩ range, and zero the instrument by enabling zero
check and then pressing the ZERO CORRECT button.

4. Select the desired range, or use auto-ranging, if desired.

5. Connect the triax cable to the INPUT jack. Keep the cable
as short as possible to minimize the effects of cable ca­pacitance. Connect the other end of the cable to the resis­tance to be measured, as shown in Figure 2-12. For
measurements above 1GΩ, it is recommended that you
use guarded connections, as described in paragraph 2.7.4.

6. Disable zero check, and allow the reading to settle.

7. T ake the reading from the display. The exponent may be
placed in either the alpha or numeric modes, as de­scribed in paragraph 2.5.

2-16

TRIAX CABLE

Operation

INPUT

ON

OFF

V, Ω GUARD

R

X

MODEL 6512

INPUT

I

RX

HI
LO

GND

AMPLIFIER

S

INPUT

R

PREAMP

OUT

COM

A. CONNECTIONS

F

V

R

B. EQUIVALENT CIRCUIT

HI

LO

(RECOMMENDED ABOVE 100MΩ)

RANGING AMPLIFIER

S

100Ω

S

R

X

MEASURED

RESISTANCE

SHIELD

CONVERTER

TO A/D

2V ANALOG

OUTPUT

Figure 2-12

Resistance measurement connections

Resistance measurement considerations

When measuring high resistance values, there are two prima­ry factors that can affect measurement accuracy and speed.
Any leakage resistance in the connecting cable or test fixture
can decrease the actual resistance seen by the instrument. Al­so, capacitance of the cable or input circuit can slow down
the response time considerably.

These two problems can be minimized by using guarding,
especially when measuring resistances above 1GΩ. Guard­ing is further discussed in paragraph 2.7.4. Noise pickup can
also be a problem, in which case the resistor must be shield­ed. Connect the shield to input low.

At low resistances, lead resistance can be a consideration.
Cancel the effects of lead resistance by shorting the ends of
the input leads and enabling suppress with zero check dis­abled. Leave suppress enabled for subsequent measure­ments.

2.7.8 Using the ohms function as a current source

The Model 6512 ohms function may also be used to generate
currents in decade values between 1nA and 100µA. To use
the instrument in this manner, simply connect the triax cable
to the INPUT jack, and connect the other end of the cable to
the circuit under test. Select the resistance range in accor­dance with the desired current (see Table 2-5). Note that cur­rent flows out from input high into input low . The test voltage
is less than 2V for all ranges 2GΩ and less, except when an
overload occurs, in which case the compliance voltage is
300V.

2-17

Operation

Table 2-5

Ohms function current output values

Ranges Output current (±1.5%)

2kΩ, 20kΩ 100µA
200kΩ 10µA
2MΩ 1µA
20MΩ 100nA
200MΩ 10nA
2GΩ, 20GΩ, 200GΩ 1nA

2.8 Analog outputs

The Model 6512 has two analog outputs on the rear panel.
The 2V ANALOG OUTPUT provides a scaled 0-2V output
with a value of 2V corresponding to full-range input. The
PREAMP OUT is especially useful in situations requiring
buffering. These tw o analog outputs are discussed in the fol­lowing paragraphs.

WARNING

When floating Input Low above 30V
RMS from earth ground, hazardous
voltage will be present at the analog out­puts. Hazardous voltage may also be
present when measuring in ohms, or
when the input voltage exceeds 30V
RMS in the volts mode.

2.8.1 2V analog output

The 2V ANALOG OUTPUT provides a scaled 0-2V output
that is inverting in the volts and ohms modes. Connections
for using this output are shown in Figure 2-13. For a full­range input, the output will be 2V ; typical examples are listed
in Table 2-6. The 2V ANALOG OUTPUT signal is not cor­rected during calibration. Gain errors of up to 3% may ap­pear at this output, depending on function and range
selection.

Note that the output impedance is 10kΩ; to minimize the ef­fects of loading, the input impedance of the device connected
to the 2V AN ALOG OUTPUT should be as high as possible.
For example, with a device with an input impedance of
10MΩ, the error due to loading will be approximately 0.1%.

Table 2-6

Typical 2V analog output values

Applied

Range

20pA 10.4pA 1.04V
2µA 1.65µA 1.65V
200mV 140mV 1.4V
200V 35V 0.35V
200kΩ 175kΩ 1.75V
20GΩ 9.5GΩ 0.95V
200pC 125pC 1.25V
20nC 19nC 1.9V

*Output values within ±3% of nominal value.

signal

Nominal 2V analog
output value*

2-18

CAUTION

Connecting PREAMP OUT, COM, or
2V ANALOG OUTPUT to earth while
floating the input may damage the in­strument.

Operation

LO

INPUT

OFF ON

V, Ω GUARD

MODEL 6512

COM

2V ANALOG OUT

RF =2MΩ (X10)

200kΩ (X1)
20kΩ (X0.1)
2kΩ (X0.01)

MODEL 1683

TEST LEAD KIT

A. CONNECTIONS

HI

MEASURING DEVICE

(EXAMPLE: CHART RECORDER)

INPUT FROM

PREAMP

200kΩ

S

MODEL 6512

Figure 2-13

Typical 2V analog output connections

2.8.2 Preamp out

The PREAMP OUT of the Model 6512 follows the signal am­plitude applied to the INPUT terminal. Some possible uses for
PREAMP OUT include buffering of the input signal, as well as
for guarding in the volts and ohms modes. Connections and
equivalent circuits for the preamp output are sho wn in Figure 2-

14. Full-range outputs for various functions and ranges are list­ed in T able 2-7. Since the PREAMP OUT signal is not correct­ed during calibration, gain errors of up to 3% may appear at this
output, depending on function and range selection. For all volts
ranges, PREAMP OUTPUT accuracy is typically 5ppm.

WARNING

High voltage may be present between the
PREAMP OUT and COM terminals de­pending on the input signal (see Table 2-

10kΩ

100Ω

S

B. EQUIVALENT CIRCUIT

2V ANALOG OUTPUT

COM

INPUT RESISTANCE OF

RL =

MEASURING DEVICE

7). Open-circuit voltage of 300V is present
at PREAMP OUT in the ohms function.

CAUTION

Connecting PREAMP OUT, COM, or 2V
ANALOG OUTPUT to earth while float­ing input may damage the instrument.

Note that the PREAMP OUT output resistance is 100 Ω . The
output resistance appears between Input Low and Analog
Output Low to keep the resistor out of the loop when using
external feedback elements. To keep loading errors under

0.1%, the device connected to the PREAMP OUT should
have a minimum input impedance of 100k Ω

.

2-19

Operation

PREAMP OUT

INPUT

OFF ON

V, Ω GUARD

MODEL 6512

HI

V

IN

LO

100Ω

S

GND

VOLTS

I

HI

R

X

LO

COM

V

OUT

PREAMP OUT

COM

S

V

OUT

PREAMP OUT

COM

= V

IN

R

= I R

A. CONNECTIONS

I

IN

L

X

Q

R

L

IN

MODEL 1683

TEST LEAD KIT

HI
LO

S

GND

HI
LO

S

HI

LO

MEASURING DEVICE

R

F

PREAMP OUT

AMPS

C

F

PREAMP OUT

100Ω

V

OUT

V

OUT

COM

= Q

COM

S

= -IINR

IN

C

F

F

R

L

R

L

S

GND

Figure 2-14

Typical preamp out connections

OHMS

100Ω

S

B. EQUIVALENT CIRCUITS

GND

100Ω

S

COULOMBS

2-20

VIZ

FB

–=

Table 2-7

Full-range PREAMP OUT values

Full-range

Function Range

Volts 200mV

2V
20V
200V

Amps 2pA, 2nA, 2µA, 2mA

20pA, 20nA, 20µA, 20mA
200pA, 200nA, 200µA

Ohms* 2k Ω

20k Ω -2G Ω
20G
200G Ω

Coulombs 200pC

2nC
20nC

*W ARNING: Open-circuit voltage of 300V present at PREAMP OUT in ohms.

value

200mV
2V
20V
200V

200mV
2V
20V

200mV
2V
20V
200V

200mV
2V
20V

Operation

ohms). The maximum voltage span in external feedback
is

±20V.

2. The input impedance in the external feedback mode is
given by the relationship Z

IN

= Z

FB

/A

, where: Z

V

FB

is
the impedance of the external feedback network, and A
is the open-loop gain of the electrometer (typically

6

greater than 10
= 10M Ω || Z

). Note that the input impedance is Z

when zero check is enabled.

FB

IN

3. The voltage at the PREAMP OUT terminal is given by
the formula:

where Z

is the value of the feedback impedance.

FB

4. Any feedback elements should be housed in a suitable
shielded enclosure (see paragraph 2.9.2 below). Insula­tors connected to Input HI should be made of Teflon or
other high-quality insulating material and should be
thoroughly cleaned to maintain the high input imped­ance and low input current of the Model 6512. If these
insulators become contaminated, they can be cleaned
with methanol and then dried with clean, pressurized air.

C

FB

V

2.9 Using external feedback

The external feedback function provides a means to extend
the capabilities of the Model 6512 Electrometer to such uses
as logarithmic currents, non-decade current ranges, as well
as non-standard coulombs ranges. The following paragraphs
discuss the basic electrometer input circuitry and methods to
implement these functions.

2.9.1 Electrometer input circuitry

A simplified diagram of the electrometer input in the external
feedback mode is shown in Figure 2-15. An input current ap­plied to the inverting (-) input of the op amp is nulled by a
current fed back through the internal feedback network made
up of R
pears at the PREAMP OUT, this internal network can be re­placed by an external network connected between the
preamp output and Input HI connections. When using exter­nal feedback, the following factors must be taken into ac­count:

and C

FB

. Because the output of the op amp ap-

FB

R

FB

ZERO CHECK

HIGH

INPUT
LOW

COM

PREAMP
OUT

OP AMP

100MΩ

S

100Ω

(CHASSIS)

TO RANGING

AMPLIFIER

Figure 2-15

Electrometer input circuitry (external feedback mode)

1. The maximum current value that can be supplied by the
preamp output is 20mA in amps (1mA in volts and

2-21

Operation

2.9.2 Shielded fixture construction

Since shielding is so critical for proper operation of external
feedback, it is recommended that a shielded fixture similar to
the one shown in Figure 2-16 be used to house the feedback
element. The fixture is constructed of a commercially avail­able shielded fixture modified with the standard BNC con­nectors replaced with triaxial female connectors. For
convenience, a banana jack can be mounted on the box to
make the necessary PREAMP OUT connection. Alternately,
a wire could be run through a rubber grommet mounted in a
hole in the side of the box. Note that input low is connected
to chassis ground within the shielded box. This connection
can be made by using a small solder lug secured with a
screw.

2.9.3 External feedback procedure

Use the following procedure to operate the Model 6512 in
the external feedback mode.

LO

SOLDER LUG

5

2

1. Connect the feedback element between the PREAMP
OUT terminal and the Input High terminal.

2. Place the instrument in the external feedback mode by
pressing the SHIFT then VOLTS buttons in that order.
The AMPS and V OLTS indicators will illuminate simul­taneously in the external feedback mode.

3. The display will show the voltage measured at the out­put of the input preamplifier (PREAMP OUT). Howev­er, the “V” exponent will not appear as in the volts
mode. For example, with a 150mV output the display
will show:

150.00 m

(External feedback may be temporarily digitally cali­brated as outlined in paragraph 7.4.16.)

4. The external feedback mode may be cancelled by press­ing one of the four functions keys (VOLTS, OHMS,
COUL, or AMPS).

INPUT LOW (INNER SHIELD)

1

INPUT HIGH (CENTER CONDUCTOR)

2

4

TO 6512 INPUT JACK

TO PREAMP OUT

FEEDBACK

ELEMENT

HI

LO

GND

237-ALG-2

CABLE

ITEM DESCRIPTION MFR. PART NUMBER

1 SHIELDED FIXTURE POMONA #2390
2 FEMALE TRIAXIAL KEITHLEY 7078-TRX-TBC
3 BANANA JACK KEITHLEY BJ-9-2
4 TRIAXIAL CABLE KEITHLEY 237-ALG-2
5 TRIAXIAL CABLE KEITHLEY 7078-TRX-3

Figure 2-16

Shielded fixture construction

SHIELDED

FIXTURE

PARTS LIST

FROM SIGNAL

HI

FEEDBACK ELEMENT

3

A. CONSTRUCTION

PREAMP OUT

7078-TRX-3

CABLE

B. EQUIVALENT CIRCUIT

HI
LO

GND

S

6512 INPUT AMP

TO RANGING

AMP AND A/D

2-22

Operation

2.9.4 Non-standard coulombs ranges

In its standard form, the Model 6512 has three coulombs
ranges allowing it to measure charge between 10fC and
20nC. Different charge measurement ranges can be used by
placing an external feedback capacitor between the
PREAMP OUT and Input HI and then placing the instrument
in the external feedback mode.

Charge is related to capacitance and voltage by the formula:
Q = CV, where Q is the charge in coulombs, C is the capaci­tance in farads, and V is the v oltage in volts. The Model 6512
display will read charge directly in units determined by the
value of C. For example, a 1µF capacitor will result in a dis­played reading of 1µC/V.

In practice, the feedback capacitor should be greater than
100pF for feedback stability and of suitable dielectric mate­rial to ensure low leakage and low dielectric absorption.
Polystyrene, polypropylene, and Teflon dielectric capacitors
are examples of capacitor types with these desirable charac­teristics. The capacitor should be mounted in a shielded fix­ture like the one in Figure 2-16.

To discharge the external feedback capacitor, enable zero
check. The discharge time constant will be given by: τ =
(10M Ω ) (C

). Allow five time constants for discharge to

FB

within 1% of final value.

2.9.5 Logarithmic currents

The use of a diode junction in the external feedback path per­mits a logarithmic current-to-voltage conversion. This rela­tionship for a junction diode is given by the equation:

V = mkT/q ln(I/I

where: q = unit of charge (1.6022 × 10

k = Boltzmann’s constant (1.3806 × 10
T = temperature (K).

The limitations in this equation center on the factors I
and R

. I

is the extrapolated current for V

B

O

proportional constant, m, accounts for the different character
current conduction (recombination and diffusion mecha­nisms) within the junction, typically varying between 1 and

2. Finally, R

constitutes the ohmic bulk resistance of the di-

B

) + IR

O

B

-19

)

. An empirical

O

-23

)

, m,

O

ode junction material. I

and R

O

limit the usefulness of the

B

junction diode at low and high currents respectively. The fac­tor m introduces non-linearities between those two extremes.
Because of these limitations, most diodes have a limited
range of logarithmic behavior.

A solution to these constraints is to use a transistor config­ured as a “transdiode” in the feedback path, as shown in Fig­ure 2-17. Analyzing the transistor in this configuration leads
to the relationship:

V = kT/q[ln(I/I

where h

is the current gain of the transistor.

FE

From this equation, proper selection of Q
device with high current gain (h

) - ln(h

O

/(1 + h

FE

), which is maintained

FE

))]

FE

would require a

1

over a wide range of emitter currents. Suitable devices for
this application include Analog Devices AD812 and Preci­sion Monolithics MAT-01. Use the enclosure in Figure 2-16
to shield the device.

Frequency compensation/stabilization is accomplished by
adding a feedback capacitor, C

. The value of this capacitor

FB

depends on the particular transistor being used and the max­imum current level expected. Compensation at maximum
current is required because the dynamic impedance will be
minimum at this point. It should be noted that the response
speed at lower currents will be compromised due to the in­creasing dynamic impedance, which is given by the follow­ing formula:

Z

dV

-------- kT==

dI

/qI = 0.026/I (@ 25°C)

Using the above transistors, a minimum RC time constant of
100µsec at maximum input current would be used. At I

IN

(max) of 100µA, this value would correspond to 0.4µF. Note
that at 100nA, this value would increase the RC response
time constant to 100msec. A minimum capacitance of 100pF
is recommended.

Although the input signal to this particular circuit is assumed
to be a current, conversion to voltage input could be per­formed by placing a shunt resistor across the input. However ,
the nominal voltage burden of 1mV must be considered as an
error signal that must be taken into account.

2-23

Operation

MODEL 6512

INPUT

10MΩ

PREAMP

OUT

HI

LO

100Ω

(CHASSIS)

CURRENT

INPUT

COM

Q1

Figure 2-17

“Transdiode” logarithmic current configuration

Further processing of the current response can be achieved
by using the suppress feature. For example, suppress could
be enabled with a reference input current applied. For all
subsequent currents, the natural logarithm of the ratio of the
measured current to the suppressed current would then be
displayed:

V
I

DISP

))

O

= V

SUPPRESS

= kT/q (ln (I
= 0.26/I (ln (I

kT/q (ln (I

/I

READ

SUPPRESS

/I

READ

SUPPRESS

READ

))

/I

) - ln (I

O

)) @ 25°C

SUPPRESS

NOTE

The circuit topology of Figure 2-17 works
for positive input currents only. For bipo­lar input signals, an external offset bias
must be applied, or use a PNP transistor
for Q

.

1

ZERO
CHECK

S

TO RANGING
AMPLIFIER

OP AMP

S

NOTE: PRESS SHIFT VOLTS TO ENTER
EXTERNAL FEEDBACK MODE

2.9.6 Non-decade current gains

The Model 6512 electrometer input uses internal decade re­sistance feedback networks for the current ranges. In some
applications, non-decade current gains may be desirable. As
shown in Figure 2-18, an external feedback resistor , R
be used to serve this purpose. Limitations on the magnitude
of the feedback current require that the value of R

/

er than 10

Note that external feedback can be temporarily calibrated
over a range of ±12% using the calibration program with the
calibration jumper in the disable position. See paragraph

7.4.16 in Section 7.

2

Ω .

be great-

FB

FB

, can

2-24

CURRENT

INPUT

Figure 2-18

Non-decade current gains

INPUT

HI
LO

PREAMP

OUT

COM

R

FB

10MΩ

100Ω

(CHASSIS)

ZERO
CHECK

OP AMP

S

NOTE: PRESS SHIFT VOLTS TO ENTER
EXTERNAL FEEDBACK MODE

RANGING
AMPLIFIER

2.10 Using zero correct and

CIN = 20pF

10MΩ

INPUT

C

IN

Z

F

CIN = 20pF

INPUT

C

IN

VOLTAGE

AMPS

CIN = 20pF

10MΩ

INPUT

C

IN

1000pF

COULOMBS

10MΩ

INPUT

C

IN

Z

F

I

N

900kΩ

OHMSCIN = 20pF

10MΩ

100Ω (mA)
100kΩ || 1000pF (µA)
100MΩ || 220pF (nA)
100GΩ || 5pF (pA)

ZF =

100kΩ || 1000pF (ALL kΩ, 2MΩ)
100MΩ || 22pF (20MΩ, 200MΩ,
ALL GΩ)

ZF =

baseline suppression

The Model 6512 has zero correction and baseline suppres­sion modes that allow the cancellation of any internal offsets,
or the storage of a baseline value that can be subtracted from
subsequent readings.

2.10.1 Zero correct and zero check

The ZERO CORRECT and ZER O CHECK b uttons work to­gether to cancel any internal offsets that might reduce accu­racy. Note that the specifications listed for the instrument at
the front of this manual assume that the instrument has been
zeroed; use the following procedure to zero the instrument.
Note that the instrument should be zero corrected on the
range to be used, or on the lowest range of the function being
used.

Operation

Figure 2-19

Equivalent input impedance with zero check enabled

Proceed as follows to zero correct the instrument:

1. With the zero correct mode off, press the ZERO
CHECK button. Be sure the ZERO CHECK light is on.
In this mode, the input signal is disconnected from the
input amplifier, and the input circuit is configured. as
shown in Figure 2-19. The internal preamplifier is con­figured to measure its own offset when zero check is en­abled.

2. Press the ZERO CORRECT button to zero the instru­ment. Note that if zero check is not enabled, the zeroing
process will not take place. The previously stored zero
parameter will be used instead.

3. To take readings, press ZERO CHECK to disable the
zero check mode.

4. Readings can now be taken in the normal manner. Note
that the instrument will remain zeroed even if the instru­ment is moved uprange.

5. For maximum accuracy, the zero correction process
should be repeated every 24 hours when the ambient
temperature changes by more than 1°C, or when the
function is changed.

NOTES:

1. Leave zero check enabled when connecting or discon­necting input signals (except for the coulombs func­tion), or when changing functions.

2. Zero will automatically be scaled when the instrument is
moved uprange.

3. Do not move the instrument down range after zero cor­rection. Re-zero the instrument after moving down range.

2.10.2 Using suppression

The suppression mode allows a stored offset v alue to be sub­tracted from subsequent readings. When the SUPPRESS
button is pressed, the instrument will trigger a conversion
and internally store the displayed value as a baseline. The
SUPPRESS LED will illuminate to indicate that the suppres­sion mode is enabled. All subsequent readings will be the
difference between the suppressed value and the actual sig­nal level.

2-25

Operation

The baseline maintains its absolute value regardless of range.
For example, if a 1V signal is suppressed on the 2V range, it
will remain at 1V on the 20V and 200V ranges. Only one
reading for the presently selected function can be sup­pressed; the value will be lost if the function is changed.

The suppressed readings can be as small as the resolution of
the instrument will allow , or as lar ge as full range. Some typ­ical examples include:

Suppressed Applied Displayed
Reading Signal Value

+10.500V +18.600V +8.100V
+2.556nA +1.8000nA -0.7560nA

-12.600mA +4.500mA +17.100mA

To use suppression, perform the following steps:

1. Cancel suppress if presently enabled.

2. Select a range and function that is consistent with the an­ticipated measurement.

3. Connect the signal to be suppressed to the instrument input.

WARNING

With suppress enabled, the voltage on
the input terminals may be significantly
larger than the displayed value. For ex­ample, if a 150VDC baseline is stored,
an applied voltage of +175V will result
in a displayed value of only +25V.

NOTES:

1. Using suppress reduces the dynamic range of the mea­surement. For example, if the suppressed value is
100mV on the 200mV range, an input voltage of 100mV
or more will over-range the instrument even though in­put voltages up to 199.99mV are normally within the ca­pabilities of the 200mV range. If the instrument is in the
auto-range mode, it will move up range, if necessary.

2. Setting the range lower than the suppressed value will
over-range the display; the instrument will display the
“OL” message under these conditions.

3. To store a new baseline, suppress must first be disabled
and then enabled once again. The new value will be
stored with the first triggered conversion.

4. Do not move the instrument down range when using
suppress.

2.11 Data storage

The Model 6512 has an internal 100-point data store mode
that can be used to log a series of readings. The fill rate of the
data store buffer can be set to specific interv als by a parame­ter that is entered when the storage mode is first enabled. Al­ternatively, a special one-shot trigger mode can be used to
control the fill rate from the front panel. Once data is stored,
readings can be easily recalled from the front panel.

Minimum and maximum values can also be retained for fu­ture recall. As long as data store is enabled, maximum and
minimum values are updated with each reading conversion.

4. Press the SUPPRESS button. The triggered reading will
be stored at that point.

5. Disconnect the suppressed signal from the input, and
connect the signal to be measured in its place. Subse­quent readings will be the difference between the sup­pressed value and the applied signal.

6. To return the instrument to the normal mode, press the
SUPPRESS button. The SUPPRESS light will go off,
and the instrument will be taken out of the suppression
mode. The previously stored suppressed value will be
cancelled.

2-26

Enter the data storage mode as follows:

1. Press and hold the DATA STORE ON/OFF button. The
instrument will then scroll through the various reading
rates that are listed in Table 2-8. In addition to the con­tinuous rate, which stores readings at the conversion rate
(about three readings per second), five additional inter­vals from one reading per second to one reading per
hour are available. A special trigger mode allows you to
control the interval with the TRIG button. During the
rate selection process, the display will appear as fol­lows:

r=3

In this example, the rate parameter is 3, indicating a 1
rdg/min interval.

Operation

Table 2-8

Data store reading rates

r Value Rate

0
1
2
3
4
5
6

2. To select the desired interval, simply release the ON/
OFF button when the desired rate appears in the display .
The Model 6512 will then begin storing readings at the
selected rate. If you selected the triggered mode, one
reading will be stored in memory each time the front
panel TRIG button is pushed.
(For rapid starts, the rate can be pre-selected by pressing
ON/OFF, releasing the button when the selected rate is
displayed, and then turning off data store. Storage will
then begin at the pre-selected rate the next time the ON/
OFF button is pressed.)

3. When data store memory is full (after all 100 readings
have been stored), the instrument will stop logging data,
and the DAT A LED will flash to indicate that memory is
full.

4. Readings can be recalled any time (even if the instru­ment is still logging) by pressing and holding the RE­CALL button. Holding the RECALL button in causes
the reading memory location number to be displayed.
Releasing the RECALL button causes the reading at that
location to be displayed. The first data point to be dis­played will be the last reading stored. For example, if
reading #65 was the last point, the display will show:

5. The second and third points will be the high and low
data points. For example, for the high value, the display
will show:

Similarly, the display will show the following for the
low data point:

Conversion Rate (every 360msec)
One Reading Per Second
One Reading Every 10 seconds
One Reading Per Minute
One Reading Every 10 Minutes
One Reading Per Hour
One reading Per Front Panel Trigger

n=65

n=HI

n=Lo

6. Following these three points, the remaining data points
will be displayed, beginning with the first one stored.
The memory location will increment from 1 to the max­imum point stored. For example, the tenth reading ap­pears as:

n=10

7. T o continue recalling readings, use the RECALL b utton
to scroll reading memory locations. Scrolling becomes
more rapid if the RECALL button is held in. Release the
button when the desired data point is displayed.

8. The recall mode can be cancelled simply by pressing
SHIFT RECALL. The instrument will then return to the
normal display mode. As long as data store is not dis­abled and then re-enabled, readings are retained within
memory. You can return to the recall mode at any time to
review data.

9. To cancel data store operation, press the ON/OFF but­ton. The ON/OFF LED will turn off, indicating that data
store is disabled. Data is retained until data store is en­abled once again. Thus, you can still recall data even af­ter data store is turned off.

Data Store Operating Notes:

1. Data logging continues at the selected rate during recall
until all 100 locations have been filled. Logging stops
when all 100 locations are full, as indicated by the flash­ing DATA indicator.

2. The data store trigger mode should not be confused with
the front panel trigger mode. The data store trigger mode
is enabled by entering the special trigger parameter
(r=6) at the beginning of the data storage process, while
the front panel trigger mode is entered by pressing
SHIFT TRIG.

3. If the instrument is placed in the front panel one-shot
trigger mode, display readings will be triggered at the
data store rate interval except when r=0. For e xample, if
the instrument is set up for 10 minute intervals, one
reading will be triggered and displayed every 10 min­utes. When r=0, a single reading is stored each time an
appropriate trigger is received (for example, GET in the
T3 trigger mode, as described in Section 3).

4. The data store rate can be used to control the data output
rate in the IEEE- 488 talk-only mode. To use the Model
6512 in this manner, place the instrument in the talk­only mode (see Section 3). Now enter the data storage
mode, and select the desired interval as described above.

2-27

Operation

The instrument will then output readings over the IEEE­488 bus at the selected rate.

5. The storage rate in r=0 and r=1 may be affected if the in­strument is in auto-range and a range change occurs.
Typically, it takes about 350msec per range change.

Minimum/maximum operation

Minimum/Maximum operation is essentially separate from
data store except for the fact that both are enabled or disabled
by the ON/OFF button. Thus, the minimum and maximum
data points are continuously updated with each triggered
conversion as long as the ON/OFF LED is on. Note that only
range, not function, is indicated when reading maximum and
minimum values. Maximum and minimum values can be ob­tained during the recall process, as indicated in step 4 above.

2.12 External triggering

The Model 6512 has two BNC connectors on the rear panel
associated with instrument triggering. The EXTERNAL
TRIGGER INPUT allows the instrument to be triggered by
other devices, while the METER COMPLETE OUTPUT al­lows the instrument to trigger other devices.

2.12.1 External trigger input

The Model 6512 may be triggered on a continuous or one­shot basis. For each of these modes, the trigger stimulus will
depend on the selected trigger mode, which is further de­scribed in Section 3. In a continuous trigger mode, the instru­ment takes a continuous series of readings. A trigger
stimulus in continuous aborts a reading in progress and trig­gers a new series of readings. In a one-shot mode, only a sin­gle reading is taken each time the instrument is triggered.

ternal trigger mode, it will be triggered to take readings while
in either a continuous or one-shot mode when the negative­going edge of the external trigger pulse occurs.

TRIGGERS ON

LEADING EDGE

TTL HIGH

(2V-5V)

TTL LOW

(≤0.8V)

10µs

MINIMUM

Figure 2-20

External trigger pulse specifications

To use the external triggering, proceed as follows:

1. Connect the external trigger source to the rear panel
BNC EXTERNAL TRIGGER INPUT connector. The
shield (outer) part of the connector is connected to digi­tal common. Since an internal pull-up resistor is includ­ed, a mechanical switch may be used. Note, however,
that de-bouncing circuitry will probably be required to
avoid improper triggering.

2. Place the instrument in the one-shot trigger mode by
pressing SHIFT and then TRlG in that order . The instru­ment will indicate that it is in the one-shot mode by illu­minating the SGL indicator.

3. To trigger the instrument, apply a pulse to the EXTER­NAL TRIGGER INPUT. The instrument will process a
single reading each time the pulse is applied. Note that
the instrument may also be triggered by pressing TRIG.

4. To return the instrument to the continuous mode, press
the SHIFT and TRIG buttons in sequence.

The EXTERNAL TRIGGER INPUT requires a falling-edge
pulse at TTL logic levels, as shown in Figure 2-20. The low
logic level should be between 0-0.8V, and the high level
should be 2-5V. The minimum pulse width for reliable trig­gering is approximately 10µsec. Connections to the rear pan­el EXTERNAL TRIGGER INPUT jack should be made with
a standard BNC coaxial cable. If the instrument is in the ex-

2-28

NOTES:

1. External triggering can be used to control the fill rate in
the data store mode. See paragraph 2.11 for details.

2. The Model 6512 must be in the appropriate trigger mode
to respond to external triggering (the unit will be in this
mode upon power-up). See Section 3 for details on pro­gramming trigger modes.

Operation

3. If a trigger overrun occurs (the instrument is triggered
while processing a reading from a previous trigger), it
will ignore the trigger and display the following:

t Err

2.12.2 Meter complete output

The Model 6512 has an available output pulse that can be used
to trigger other instrumentation. A single TTL-compatible
negative-going pulse with a minimum duration of 10µsec (see
Figure 2-21) will appear at the METER COMPLETE OUT­PUT jack each time the instrument completes a reading. To
use the METER COMPLETE OUTPUT , proceed as follo ws:

1. Connect the Model 6512 to the instrument to be triggered
with a suitable shielded cable. Use a standard BNC coax­ial cable to make the connection to the Model 6512.

CAUTION

Do not exceed 30V RMS between the
METER COMPLETE common (outer
ring) and chassis ground, or instrument
damage may occur.

2. Select the desired function, range, trigger mode, and
other operating parameters, as desired.

3. In a continuous trigger mode, the instrument will output
pulses at the conversion rate; each pulse will occur after
the Model 6512 has completed a conversion (about ev­ery 360msec).

4. In a one-shot trigger mode, the Model 6512 will output
a pulse once each time it is triggered after it completes
the reading conversion.

10µs

MINIMUM

BEGIN NEXT

CONVERSION

READING

LS TTL HIGH

(3.4V TYPICAL)

LS TTL LOW

(0.25V TYPICAL)

DONE

2.12.3 T riggering example

As an example of using both the external trigger input and
the meter complete output, assume that the Model 6512 is to
be used in conjunction with a Keithley Model 7001 Switch
System to allow the Model 6512 to measure a number of dif­ferent signals, which are to be switched by the scanner. Us­ing appropriate scanner cards, the Model 7001 can switch up
to 80 2-pole channels.

By connecting the triggering inputs of the two instruments
together, a complete automatic measurement sequence could
be performed. Data obtained from each measurement point
could be stored by the data store mode of the Model 6512.
Alternatively, the Model 6512 could be connected through
the IEEE-488 bus to a printer, which w ould print out the data
for each point as it is measured.

Once the Model 7001 is programmed for its scan sequence,
the measurement procedure is set to begin. When the Model
7001 closes the selected channel, it triggers the Model 6512
to take a reading. When the Model 6512 finishes the reading,
it triggers the Model 7001 to scan to the next channel. The
process repeats until all channels have been scanned.

T o use the Model 6512 with the Model 7001, proceed as follows:

1. Connect the Model 6512 to the Model 7001 as shown in
Figure 2-22. Use shielded cables with BNC connectors.
The Model 6512 METER COMPLETE OUTPUT jack
should be connected to the Model 7001 EXTERNAL
TRIGGER input jack. The Model 6512 EXTERNAL
TRIGGER INPUT should be connected to the Model
7001 CHANNEL READY output. Additional connec­tions, which are not shown on the diagram, will also be
necessary to apply signal inputs to the scanner cards, as
well as for the signal lines between the scanner and the
Model 6512.

2. Place the Model 6512 in the one-shot trigger mode by
pressing the SHIFT and TRIG buttons, in that order.

3. Install the desired scanner cards and make the required
input and output signal connections.

4. Program the Model 7001 scan list and trigger modes as re­quired. See the Model 7001 Instruction Manual for details.

5. If data storage is required, enter the data storage mode
as described in paragraph 2.11.

Figure 2-21

Meter complete pulse specifications

2-29

Operation

MODEL 6512

MODEL 7001

CHANNEL

READY

Figure 2-22

External triggering example

6. Begin the measurement sequence by pressing the Model
7001 STEP button. The Model 7001 will close the first
channel and trigger the Model 6512 to take a reading.
When the Model 6512 completes the reading, it will
trigger the Model 7001 to go to the next channel. The
process repeats until all programmed channels have
been scanned.

2.13 Measurement considerations

The Model 6512 is a highly sensitive instrument that can
measure extremely low signal lev els. At these lo w signal lev­els, a number of factors can affect a measurement. Some con­siderations when making measurements with the Model
6512 are discussed in the following paragraphs.

2.13.1 Ground loops

Ground loops that occur in multiple-instrument test set-ups
can create error signals that cause erratic or erroneous mea­surements. The configuration shown in Figure 2-23 introduc­es errors in two ways. Large ground currents flowing in one
of the wires will encounter small resistances, either in the
wires, or at the connecting points. This small resistance re­sults in voltage drops that can affect the measurement. Even
if the ground loop currents are small, magnetic flux cutting
across the large loops formed by the ground leads can induce
sufficient voltages to disturb sensitive measurements.

EXTERNAL

TRIGGER

METER

COMPLETE

OUTPUT

SIGNAL LEADS

INSTRUMENT

A

TYPICAL GROUND LOOP

CAUSES CURRENT FLOW

IN A SIGNAL LEAD

POWER LINE GROUND

EXTERNAL

TRIGGER

INPUT

INSTRUMENT

B

INSTRUMENT

Figure 2-23

Multiple ground points create a ground loop

T o prev ent ground loops, instruments should be connected to
ground at only a single point, as shown in Figure 2-24. Note
that only a single instrument is connected directly to power
line ground. Experimentation is the best way to determine an
acceptable arrangement. For this purpose, measuring instru­ments should be placed on their lowest ranges. The configu­ration that results in the lowest noise signal is the one that
should be used.

INSTRUMENT

A

POWER LINE GROUND

INSTRUMENT

B

INSTRUMENT

Figure 2-24

Eliminating ground loops

C

C

2-30

Operation

2.13.2 EIectrostatic interference

Electrostatic interference occurs when an electrically
charged object is brought near an uncharged object, thus in­ducing a charge on the previously uncharged object. Usually,
effects of such electrostatic action are not noticeable because
low impedance levels allow the induced charge to dissipate
quickly. Ho we ver, the high impedance levels of many Model
6512 Electrometer measurements do not allow these charges
to decay rapidly, and erroneous or unstable readings may re­sult. These erroneous or unstable readings may be caused in
the following ways:

1. DC electrostatic field can cause undetected errors or
noise in the reading.

2. AC electrostatic fields can cause errors by driving the in­put preamplifier into saturation, or through rectification
that produces DC errors.

Electrostatic interference is first recognizable when hand or
body movements near the experiment cause fluctuations in
the reading. Pick-up from AC fields can also be detected by
observing the electrometer preamp output on an oscillo­scope. Line frequency signals on the output are an indication
that electrostatic interference is present.

Means of minimizing electrostatic interference include:

1. Shielding. Possibilities include: a shielded room, a
shielded booth, shielding the sensitive circuit, and using
shielded cable. The shield should always be connected
to a solid connector that is connected to signal low. If
circuit low is floated above ground, observe safety pre­cautions, and avoid touching the shield. Meshed screen
or loosely braided cable could be inadequate for high
impedances, or in strong fields. Note, however, that
shielding can increase capacitance in the measuring cir­cuit, possibly slowing down response time.

2. Reduction of electrostatic fields. Moving power lines or oth­er sources away from the experiment reduces the amount of
electrostatic interference seen in the measurement.

2.13.3 Thermal EMFs

Thermal EMFs are small electric potentials generated by dif­ferences in temperature at the junction of two dissimilar met­als. Although thermal EMFs are most troublesome with lo w­voltage signals, they can also affect measurements made at
higher levels in extreme cases.

Low-thermal connections should be used whenever thermal
EMFs are known to be a problem. Crimped copper-to-copper

connections should be used to minimize these effects. Make
certain that all connecting surfaces are kept clean and free of
oxides, since copper-to-copper oxide junctions generate
much higher thermal EMFs than do pure copper-to-copper
connections.

2.13.4 Electromagnetic interference (EMI)

The electromagnetic interference characteristics of the Mod­el 6512 Electrometer comply with the electromagnetic com­patibility (EMC) requirements of the European Union as
denoted by the CE mark. However, it is still possible for sen­sitive measurements to be affected by external sources. In
these instances, special precautions may be required in the
measurement setup.

Sources of EMI include:

• radio and television broadcast transmitters

• communications transmitters, including cellular phones
and handheld radios

• devices incorporating microprocessors and high speed
digital circuits

• impulse sources as in the case of arcing in high-voltage
environments

The effect on instrument performance can be considerable if
enough of the unwanted signal is present. The effects of EMI
can be seen as an unusually large offset, or , in the case of im­pulse sources, erratic variations in the displayed reading.

The instrument and experiment should be kept as far away as
possible from any EMI sources. Additional shielding of the
instrument, experiment, and test leads will often reduce EMI
to an acceptable level. In extreme cases, a specially con­structed screen room may be required to sufficiently attenu­ate the troublesome signal.

External filtering of the input signal path may be required. In
some cases, a simple one-pole filter may be sufficient. In
more difficult situations, multiple notch or band-stop filters,
tuned to the offending frequency range, may be required.
Connecting multiple capacitors of widely different values in
parallel will maintain a low impedance across a wide fre­quency range. Such filtering, howev er , may ha ve detrimental
effects (such as increased response time) on the measure­ment.

2.13.5 Leakage resistance effects

At normal resistance levels, the effects of leakage resistance
are seldom seen because any leakage resistance present is
generally much higher than the resistance levels encountered

2-31

Operation

in the circuit under test. At the high resistance le vels of man y
Model 6512 measurements, however, leakage resistance can
have a detrimental effect on the measurement. Such leakage
resistance can occur in the circuit under test (on PC boards,
for example), in the connecting cable, or even at the elec­trometer input itself, especially if the input connector is not
kept clean.

T o see how leakage resistance can affect measurement accu­racy, let us review the equivalent circuit in Figure 2-25. E
and RS are the source voltage and source resistance respec­tively. The leakage resistance is represented by RL, while the
voltage, as seen by the electrometer, is VM.

R

S

VM =

V

M

ES R

RS + R

L

L

E

SR

L

Figure 2-25

Leakage resistance effects

and make sure that the circuit under test and connectors are
kept free of contamination.

Even with these steps, howev er , there is a limit as to how high
the leakage resistance can be. In those cases, guarded input
connections should be used, as described in paragraph 2.7.4.

2.13.6 Input capacitance effects

S

Virtually any circuit has at least some small amount of dis­tributed capacitance that can slow do wn the response time of
high-impedance measurements. Even if the circuit itself has
minimal capacitance, cable or instrument input capacitance
effects can be noticeable.

As an example, assume that the Model 6512 is being used to
measure the value of a high-impedance voltage source, as
shown in Figure 2-26. The source and source resistance are
represented by ES and RS, the input capacitance is CIN, and
the voltage measured by the electrometer is VM.

R

S

E

S

C

V

IN

M

RS and RL form a voltage divider that attenuates the input
signal in accordance with the formula:

ESE

---------------------=

RSR

+

L

L

V

M

Thus, if RL has a value of 100GΩ, and RS is 10GΩ, the ac­tual voltage measured by the electrometer with a 10V source
would be:

10 100GΩ×

------------------------------------------- -=

V

M

10GΩ 100GΩ+

VM = 9.09V

Thus, we see that the effects of leakage resistance can be sub­stantial, resulting in an error of more than 9% in this case.

Certain steps can be taken to ensure that the effects of leak­age resistance are minimal. The most obvious remedy to en­sure that the leakage resistance itself is as high as possible.
Use only good quality triaxial cable for signal connections,

A. CIRCUIT

0.632 E

S

RSC

IN

B. EXPONENTIAL RESPONSE

Figure 2-26

Input capacitance effects

When ES is first applied, the voltage across the capacitance
(and thus, at the electrometer input) does not instantaneously
rise to its final value. Instead, the capacitance charges expo­nentially in accordance with the following formula:

t–

--------



V

MES

=

RC

1e

–



Note that RS is given in megohms, C is in microfarads, while
t is in seconds.

2-32

Operation

TO RANGING

AMPLIFIER

E

NOISE

E

OS

E

S

C

S

C

F

R

F

Because of the charging of CIN, the electrometer follows the
exponential curve shown in Figure 2-26B. At the end of one
time constant (RSCIN), the voltage will reach approximately
63% of its final value. At the end of two time constants
(2RSCIN), the voltage will reach about 86% of its final value,
and so on. Generally, at least fiv e time constants should be al­lowed for better than 1% accuracy.

The amount of time that must be allowed will, of course, de­pend on the relative values of RS and CIN. For example,
when measuring a voltage with a source resistance of 10GΩ
with an input capacitance of 100pF, a time constant of one
second results. Thus, at least five seconds must be allowed to
achieve a better than 1% accuracy figure. Table 2-9 summa­rizes voltage values and percentage error values for ten dif­ferent time constants (τ = R

SCIN

).

Table 2-9

Voltage and percent error for various time constants

Time
constant* V

τ
2τ
3τ
4τ
5τ
6τ
7τ

*τ = RSC

IN

M

0.632E

0.86E

S

0.95E

S

0.982E

0.993E

0.9975E

0.999E

S

S
S

S

%Error

37%
14%
5%

1.8%

0.674%

0.25%

S

0.09%

The most obvious method to minimize the slowing effects of
input capacitance is to minimize the amount of capacitance
in the circuit. Using low-capacitance cable and keeping the
cable as short as possible are two ways to do so. However,
there is a limit to the amount of capacitance reduction that
can be achieved. In those cases, especially where high im­pedance levels are involved, guarded operation (see para­graph 2.7.4) may be necessary.

Thus, if RS has a value of 10MΩ, and CIN has a value of
100pF, the half-power point will be 159Hz.

2.13.7 Source resistance

As shown in Table 2-10, a minimum value of source resis­tance is recommended for each AMPS range. The reason for
this limitation can be understood by examining Figure 2-27.
CS and CF do not affect low-frequency noise and drift and
can be ignored for the purposes of this discussion.

Figure 2-27

Simplified model for source resistance and source capaci­tance

Table 2-10

Minimum source resistance

Minimum source

Range

All pA
All nA
All µA
All mA

Input amplifier noise (E
the output can be calculated as follows:

resistance

100GΩ
100MΩ
100kΩ
100Ω

) and offset (EOS) appearing at

NOISE

While input capacitance does increase rise-time, it can help
to filter out some noise present at the input by effectively re­ducing electrometer bandwidth. If we assume that all input
capacitance is lumped into a single element, the half-power
(-3dB) point of the circuit in Figure 2-26A will be:

f

3dB–

------------------------- -=

2πR

1

SCIN

OutputE

NOISE

InputE

NOISE



1

×=




Thus, it is clear that, as long as RS >> RF, Output E
put E
Output E
× Input E

. However, as RS decreases in value relative to RF,

NOISE

increases. When RF = RS, Output E

NOISE

, and the same relationship applies for EOS.

NOISE

R

F

-------+

R

S

NOISE

NOISE

= In-

= 2

2-33

Operation

The Model 6512 will typically show insignificant degrada­tion in displayed performance with the noise gain of 2 result­ing from allowing RS = RF. Typical amplifier Input E

NOISE

is about 9µV p-p over a bandwidth of 0.1-10Hz. Amplifier
EOS can be nulled by using suppress. The temperature coef­ficient of E

is <30µV/°C, and these values can be used

OS

with the above equation to determine expected displayed
noise/drift given any source resistance. Note also that the
values given in Table 2-10 for minimum source resistance
also represent the value of R

on that range.

F

2.13.8 Source capacitance

In amps, the Model 6512 is designed to accommodate up to
10,000pF input capacitance (C
will preclude problems in most test setups and allow ex­tremely long input cable lengths without inducing instability
or oscillations.

Increasing source capacitance beyond this level may in­crease noise and induce instrument instability. Again refer­ring to Figure 2-27, the noise gain of the measurement circuit
can be found as follows:

OutputE

NOISE

where,

Z

-----------------------------------------------=

F

Z

-----------------------------------------------=

S

Note that as f ➞ 0, the above equation reduces to the low-fre­quency equation discussed in paragraph 2.13.7.

The frequency range of interest is from 0.1 to 10Hz, which is
the noise bandwidth of the A/D converter. The value of CF is
5pF for pA ranges, 22pF for nA ranges, and 1,000pF for µA
ranges. Also, since the noise factor is given in volts, current
must be converted to volts. Table 2-11 summarizes equiva­lent voltage sensitivity of Model 6512 amps ranges.

in Figure 2-27). This limit

S

InputE

NOISE

R

F

2πfRFC

()

2πfRSC

()

F

R

S

S



×=




2

1+

2

1+

1

Z

F

------ -+

Z

S

Table 2-11

Equivalent voltage sensitivity of Model 6512 amps ranges

Equivalent voltage
sensitivity (µV/

Range

2pA, 2nA, 2µA, 2mA
20pA, 20nA, 20µA, 20mA
200pA, 200nA, 200µA

count)

10µV
100µV
1mV

In general, as CS becomes larger, the noise gain increases. An
application where CS may be greater than 10,000pF is leak­age measurement of capacitors. In this case, Input E

NOISE

must include the effects of the voltage source (ES) used to
bias the capacitor (any noise in the source voltage will in­crease the input noise).

When measuring leakage currents on capacitors larger than
10,000pF, stability and noise performance can be maintained
by connecting a resistor in series with the capacitor under test.
The value of this resistor should be about 1MΩ. For lar ge ca­pacitor values (>1µF), the value of the series limiting resistor
can be made lower in order to improve settling times; how­ever, values below 10kΩ are not generally recommended.

This resistor is not critical in terms of tolerance or stability.
Any carbon composition resistor will prove adequate.

2.14 Engineering units conversion

The Model 6512 is a highly sensitive instrument with wide­ranging measurement capabilities. In the amps mode, for ex­ample, the unit can detect currents as low as 0.1fA (10
At the other extreme, resistances up to 200GΩ can be mea­sured. The instrument can display its reading either in engi­neering units (such a mA) or in scientific notation (such as
10-3A). Table 2-12 lists engineering units and their equiva­lent scientific notation values.

-16

A).

2-34

Table 2-12

Engineering units conversion

Symbol Prefix Exponent

Operation

f
p
n
µ
m
k
M
G

femto­pico­nano­micro­milli­kilo­mega­giga-

10
10
10
10
10
10
10
10

-15

-12

-9

-6

-3
3
6
9

2-35

3

IEEE-488 Programming

3.1 Introduction

This section contains detailed information on programming
Model 6512 operating modes over the IEEE-488 bus. For ad­ditional bus information, refer to the following sections of
the Appendix:

• IEEE-488 bus overview: Appendix A

• General bus commands: Appendix B

• Interface function codes: Appendix C

• Example computer programs: Appendix D

• Compatibility with Model 617 Electrometer: Appendix E

3.2 Device-dependent command
programming

IEEE-488 device-dependent commands are used with the
Model 6512 to control various operating modes such as func­tion, range, trigger mode, and data format. Each command is
made up of a single ASCII letter followed by a number rep­resenting an option of that command. For example, a com­mand to control the measuring function (Volts, Ohms, Amps,
Coul) is programmed by sending an ASCII “F” followed by
a number representing the function option.

Multiple commands

the command string. Commands sent without the execute
character will not be executed at that time, but they will be
retained within an internal command buffer for execution at
the time the X character is received. If any errors occur, the
instrument will display appropriate front panel error messag­es and generate an SRQ if programmed to do so.

Electrometer commands

Commands that directly affect the electrometer section (F , R,
C, Z, N, T, and A) will trigger a reading when the command
is executed. These bus commands affect the Model 6512
much like the front panel controls.

Order of command execution

Note that commands are not necessarily executed in the or­der received; instead, they will be e xecuted in the same order
as they appear in the status word:

Function (F); Range (R); Zero Check (C); Zero Correct (Z);
Suppress (N); Trigger (T); Reading Mode (B); Data Storage
(Q); SRQ Mode (M); EOI and Bus Hold-off (K); and Termi­nator (Y).

Thus to force a particular command sequence, you would
follow each command with the execute character, as in the
example string, C1XZ1XC0X, which can be used to zero
correct the instrument.

A number of commands may be grouped together in one
string. A command string is usually terminated with an
ASCII “X” character, which tells the instrument to execute

3-1

IEEE-488 Programming

Valid command strings

Device-dependent commands can be sent either one at a
time, or in groups of several commands within a single
string. Some examples of valid command strings include:

F0X Single command string.
F0K1R0X Multiple command string.
T6 X Spaces are ignored.

Typical invalid command strings include:

H1X Invalid command, as H is not one of the instrument

commands.

F9X Invalid command option because 9 is not an option of

the F command.

If an illegal command (IDDC), illegal command option (ID­DCO), is sent, or if a command string is sent with REN false,
the string will be ignored, and an appropriate error message
will be displayed.

Default conditions

Command default conditions are summarized in Table 3-1.

Device-dependent command summary

Device-dependent commands that control the Model 6512
are listed in T able 3-2. These commands are covered in detail
in the following paragraphs.

NOTE

REN must be true when sending device­dependent commands to the instrument, or
it will ignore the command and display a
bus error message.

Controller programs

Refer to Appendix D for e xample programs that can be used
to send commands to the Model 6512.

Table 3-1

Default conditions

Default

Mode

Function
Range
Zero check
Zero correct
Suppression
Trigger
Data format
Data store
SRQ mode
EOI and bus hold-off
Terminator

* Instrument status on power-up or after receiving DCL or SDC.

value Status*

F0
R0
C1
Z0
N0
T6
G0
Q7
M00
K0
Y<CR><LF>

Volts
Auto-range
Enabled
Disabled
Disabled
Continuous, external
Prefix, no suffix
Disabled
Disabled
Both disabled
<CR><LF>

3-2

IEEE-488 Programming

Table 3-2

Device-dependent command summary

Mode Command Description Paragraph

Calibration
Value

Reading
Mode

Zero Check C0

Function F0

Data Format G0

EOI and Bus
Hold-off

Store
Calibration

SRQ M0

Baseline
SuppressionN0N1

Data Store Q0

A+nnn.nn
or
An.nnnE+n

B0
B1
B2
B3

C1

F1
F2
F3
F4

G1
G2

K0
K1
K2
K3

L1 Store Calibration Constants in NVRAM 3.2.7

M1
M2
M8
M16
M32

Q1
Q2
Q3
Q4
Q5
Q6
Q7

Calibrate Function and Range 3.2.1

Electrometer
Buffer Reading
Maximum Reading
Minimum Reading

Zero Check Off
Zero Check On

Volts
Amps
Ohms
Coulombs
External Feedback

Reading with Prefix (NDCV-1.23456E+00)
Reading without Prefix (-1.23456E+00)
Reading with Prefix and Buffer Suffix (if in B1)
(NDCV-1.234556E+00,012)

Enable both EOI and Bus Hold-off on X
Disable EOI, Enable Bus Hold-off on X
Enable EOI, Disable Bus Hold-off on X
Disable both EOI and Bus Hold-off on X

Disable SRQ
Reading Overflow
Buffer Full
Reading Done
Ready
Error

Suppression Disabled
Suppression Enabled

Conversion Rate
One Reading Per Second
One Reading Every 10 Seconds
One Reading Per Minute
One Reading Every 10 Minutes
One Reading Per Hour
Trigger Mode
Disabled

3.2.2

3.2.3

3.2.4

3.2.5

3.2.6

3.2.8

3.2.9

3.2.10

3-3

IEEE-488 Programming

Table 3-2

Device-dependent command summary (cont.)

Mode Command Description Paragraph

Range

Volts Amps Ohms Coul. Feedback

R0
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12

Trigger Mode T0

T1
T2
T3
T4
T5
T6
T7

Status W ord U0

U1

U2
Execute X Execute other device-dependent commands 3.2.14
Terminator Y<LF CR>

Y<CR LF>

Y<ASCII>

YX
Zero

Correct

Z0

Z1

Auto Auto Auto Auto Auto
200mV 2pA 2k Ω
2V 20pA 20k Ω
20V 200pA 200k Ω
200V 2nA 2M Ω
200V 20nA 20M Ω
200V 200nA 200M Ω
200V 2µA 2G Ω
200V 20µA 20G Ω
200V 200µA 200G Ω
200V 2mA 200G Ω
200V 20mA 200G Ω
Cancel auto-ranging for all functions.

Continuous, Triggered by Talk
One-shot, Triggered by Talk
Continuous, Triggered by GET
One-shot, Triggered by GET
Continuous, Triggered by X
One-shot, Triggered by X
Continuous, Triggered by External Trigger
One-shot, Triggered by External Trigger

Send Status Word: 6512FRRCZNT0BG0QMMKYY
Send Error Conditions
Send Data Conditions

Terminator = <LF> <CR>
Terminator = <CR> <LF>
Terminator = ASCII character (except A-Z)
No terminator

Zero Correct Disabled
Zero Correct Enabled

200pC 200mV
2nC 2V
20nC 20V
20nC 20V
20nC 20V
20nC 20V
20nC 20V
20nC 20V
20nC 20V
20nC 20V
20nC 20V

External

3.2.11

3.2.12

3.2.13

3.2.15

3.2.16

3-4

3.2.1 Calibration value (A)

IEEE-488 Programming

Purpose

Format

Description

Programming The proper calibration signal must be connected to the instrument before attempting calibration.

Note

Examples A1.9X Value = 1.9.

To send calibration values to the instrument when digitally calibrating the unit.
Ann.nnn

An.nnnE+n
One advanced feature of the Model 6512 is its digital calibration capabilities. Instead of the

more difficult method of adjusting a number of potentiometers, the user need only apply an ap­propriate calibration signal and send the calibration value over the bus.

If the calibration value is outside the allowed range (±6% of nominal v alue), a number error will
occur, as indicated by the following message:

n Err

Once all functions and ranges have been calibrated, permanent storage of calibration parameters
must be performed, as described in paragraph 3.2.7.

See Section 7 for complete details on calibrating the instrument, either from the front panel or
over the bus.

A1.9E-3X Value = 0.0019

3.2.2 Reading mode (B)

Purpose To control the source of the data sent over the IEEE-488 bus.

Format B0 Electrometer reading

Default Upon power-up, or after a DCL or SDC command, the instrument will be in the B0 (electrom-

Description The reading mode command parameters allow the selection of the source of data that is trans-

B1 Data store reading
B2 Maximum reading
B3 Minimum reading

eter) mode.

mitted over the IEEE-488 bus. Through this command, you hav e a choice of data from the elec­trometer, data store reading, or minimum and maximum values. Note that the commands
associated with data store are always available; however the suffix of the reading string will
show 000 if data store is disabled, as in NDCV+1.2345E+00,000. Minimum/maximum values
returned will be the last values stored, unless these parameters are requested after a DCL, in
which case unusable readings will be returned.

When in B0, normal electrometer readings will be sent. In a continuous trigger mode, readings
will be updated at the conversion rate (one reading e v ery 360msec). In B1, readings will be tak­en from consecutive data store locations, beginning with the oldest reading and progressing to
the newest reading until all readings currently stored have been read. Once all readings have
been requested, the unit will cycle back and begin again. These readings may be accessed even
if data store is still taking place.

3-5

IEEE-488 Programming

Programming See paragraph 3.2.10 for a complete description of data storage.

Note

Examples B1X Select data store reading.

3.2.3 Zero check (C)

Purpose To zero check the instrument as an aid in nulling offsets.

Format C0 Zero check off

Default Upon power-up, or after receiving a DCL or SDC command, zero check will be enabled (C1).

While data store is enabled, the maximum (most positive) and minimum (most negative) read­ings may also be requested by sending the B2 or B3 commands. Note that the maximum and
minimum values are updated at the maximum reading rate while data store is enabled.

B3X Select minimum reading.
B0X Choose electrometer reading.

C1 Zero check on

Description The zero check command works together with the zero correct command to cancel any internal

Programming 1. In general, zero check should be enabled when changing input connections (except

Notes

Examples C0X Disable zero check.

3.2.4 Function (F)

Purpose To program the operating function of the instrument.

Format F0 Volts

Default Upon power-up, or after receiving a DCL or SDC command, the instrument will be in the F0

offsets that might upset accuracy. See paragraph 3.2.16 for a complete description of the zero
correction process.

for the coulombs function).

2. After sending a zero correct command, the instrument will be ready for a new command on
reading done.

C1X Enable zero check.

F1 Amps
F2 Ohms
F3 Coulombs
F4 External Feedback

(volts) function.

3-6

Description The function command and its options allow you to select the type of measurement made by the

Model 6512, and they perform essentially the same operations as the front panel function keys.
The parameter options associated with the function command set the instrument to measure
voltage, current, resistance, charge, or external feedback.

Programming When the instrument responds to a function command, it will be ready to take a reading once

Note

Examples F1X Select amps function.

the input circuitry is properly configured.

F3X Switch to coulombs function.

3.2.5 Data format (G)

Purpose To control the format of the data the instrument sends over the IEEE-488 bus.

Format G0 Reading with prefix (NDCV-1.23456E+00)

Default Upon power-up, or after a DCL or SDC command, the instrument will be in the G0 (reading with

Description Through the use of the G command, the format of the data the instrument sends over the bus may

Programming 1. The B command affects the source of the data. See paragraph 3.2.2 for complete details.

Notes

IEEE-488 Programming

G1 Reading without prefix (-1.23456E+00)
G2 Reading with prefix and suffix (NDCV-1.23456E+00,001)

prefix) mode.

be controlled. Figure 3-1 further clarifies the general data format. Note that the prefix identifies
a normal or overflow reading as well as the measuring function. The mantissa is always 5 ½ digits,
although the most significant digit will assume a value of 2 under overload conditions.

2. In the G2 mode, the suffix will always return 000 if the instrument is in the electrometer
reading mode (B0).

Examples G2X Prefix and suffix mode.

G0X Prefix only mode.

N=NORMAL
O=OVERFLOW

NDCV ± 1.23456 E + 02, 011 <CR LF>

DCV=VOLTS
DCA=AMPS
OHM=OHMS
DCC=COULOMBS
DCX=EXTERNAL
FEEDBACK

Figure 3-1

General data format

MANTISSA

(5 1/2 DIGITS)

EXPONENT TERMINATOR

DATA STORE

LOCATION

(B1, G2 ONLY)

3-7

IEEE-488 Programming

3.2.6 EOI and bus hold-off modes (K)

Purpose To program EOI and bus hold-off modes.

Format K0 Send EOI with last byte; hold off bus until commands processed on X

K1 Do not send EOI with last byte; hold off bus until commands processed on X
K2 Send EOI with last byte; do not hold off bus on X
K3 Send no EOI with last byte; do not hold off bus on X

Default Upon power-up, or after receiving a DCL or SDC command, the K0 mode will be enabled (both

EOI and bus hold-off enabled).

Description The K command allows control over whether or not the instrument sends the EOI command at

the end of its data string, and whether or not bus activity is held off (through the NRFD line)
until all commands sent to the instrument are internally processed once the instrument receives
the X character.

The EOI line on the IEEE-488 bus provides a method to positively identify the last byte in a
multi-byte transfer sequence. Keep in mind that some controllers rely on EOI to terminate their
input sequences. In this case, suppressing EOI with the K command may cause the controller
input sequence to hang up unless other terminator sequences are used.

The bus hold-off mode allows the instrument to temporarily hold up bus operation when it re­ceives the X character until it processes all commands sent in the command string. The purpose
of the hold-off is to ensure that the front end FETs and relays are properly configured before
taking a reading. Keep in mind that all bus operation will cease–not just acti vity associated with
the Model 6512. The advantage of this mode is that no bus commands will be missed while the
instrument is processing commands previously received.

The hold-off period depends on the commands being processed. Table 3-3 lists hold-off times
for a number of different commands. Since NRFD hold-off is employed, the handshake se­quence for the X character is completed.

Table 3-3

Bus hold-off times

Commands Bus held off on X until:

L
F, R, C
Z, N
All others

NVRAM storage completed (13msec)
6512 front end configured (20msec)
Value taken (360msec)
When X is recognized

3-8

NOTE: NRFD will be held off until each byte is recognized (1-60msec in continuous trigger
mode; 1msec in one-shot trigger mode).

Examples K1X EOI disabled, hold-off enabled.

K3X Both EOI and hold-off disabled.

3.2.7 Non-volatile memory storage (L)

Purpose To store calibration constants in NVRAM.

Format L1

Description The Model 6512 uses non-volatile (NV) RAM to store calibration parameters. Once the instru-

ment has been calibrated, as described in the last paragraph, the NVRAM storage command
should be sent to permanently store these parameters. This procedure is performed by sending
the following sequence: L1X. NVRAM storage will take place when the instrument receiv es this
command over the bus. Note that NVRAM storage may be disabled by changing the position of
a calibration jumper, as described in Section 7. If the jumper is in the enabled position, all cali­bration must be properly done, or calibration of all functions and ranges will be affected.

IEEE-488 Programming

Programming Do not perform the programming example below unless actual NVRAM storage is desired. Un-

Note

Example L1X Perform non-volatile RAM storage.

less proper calibrating parameters have been previously programmed, inadvertent use of this
command could affect instrument accuracy.

3.2.8 SRQ mask (M) and status byte format

Purpose To control which instrument conditions will cause the unit to generate an SRQ (Service Re-

quest).

Format M0 SRQ disabled

M1 Reading overflow
M2 Data store memory full (100 readings)
M8 Reading done
M16 Ready to accept bus commands
M32 Error

Default Upon power-up, or after receiving a DCL or SDC command, SRQ will be disabled (M0).

Description The SRQ command controls which of a number of conditions within the Model 6512 will cause

the instrument to request service from the controller by asserting SRQ. Once an SRQ is gener­ated, the status byte can be checked to determine if the Model 6512 was the instrument that as­serted SRQ, and, if so, what conditions caused it to do so. Note that additional data and error
conditions can be checked by using the U1 and U2 commands, as described in paragraph 3.2.13.

SRQ Mask The Model 6512 uses an internal mask to determine which conditions will cause an SRQ to be

generated. Figure 3-2 shows the general format of this mask, which is made up of eight bits. The
SRQ mask has the same general format as the status byte (described below) except for the fact
that bit 6 is not used in the SRQ mask.

3-9

IEEE-488 Programming

BIT

POSITION

VALUE

B7 B6 B5 B4 B3 B2 B1 B0

0 1/0 1/0 1/0 1/0 0 1/0 1/0

DECIMAL

WEIGHTING

1=RQS BY 6512

(STATUS BYTE ONLY)

1=READY FOR NEW COMMAND

128 64 32 16 8 4 2 1

1=ERROR

1=DATA STORE FULL

1=READING DONE

1=READING
OVERFLOW

Figure 3-2

SRQ mask and status byte format

SRQ can be programmed by sending the ASCII letter “M” follo wed by a decimal number to set
the appropriate bit in the SRQ mask. Note that the instrument may be programmed for more than
one set of conditions simultaneously. To do so, simply add up the decimal bit values for the re­quired SRQ conditions. For example, to enable SRQ under reading overflo w and buf fer full con­ditions, send M3X.

Status Byte The status byte contains information relating to data and error conditions within the instrument.

The general format of the status byte (which is obtained by using serial polling) is also shown
in Figure 3-2. Note that the various bits correspond to the bits in the SRQ mask as described
above.

The bits in the status (serial poll) byte have the following meanings:

Reading Overflow (Bit 0)—Set when an over-range input is applied to the instrument. Cleared
when a non-overflowed reading is available.

Data Store Full (Bit 1)—Set when all 100 readings in the data store buffer have been stored.
Cleared by reading a stored reading over the bus (B1X).

Reading Done (Bit 3)—Set when the Model 6512 has completed the present reading conversion.
Cleared by requesting a reading over the bus.

Ready (Bit 4)—Set when the instrument has processed all previously received commands and
is ready to accept additional commands over the bus. Cleared when processing commands.

Error (Bit 5)—Set when an error condition occurs, as described above. Cleared by reading the
error word with the U1 command.

RQS (Bit 6)—Set if the Model 6512 has asserted SRQ. Cleared by serial polling the instrument.

Bits 2 and 7 are not used, and are always set to 0.

Bit 6 provides a means for you to determine if SRQ was asserted by the Model 6512. If this bit is
set, service was requested by the instrument. Bit 5 flags a Model 6512 error condition, which can
be further checked with the U1 command. If this bit is set, one of the following errors has occurred:

3-10

IEEE-488 Programming

1. An illegal device dependent command (IDDC) or illegal device dependent command op­tion (IDDCO) was transmitted.

2. The instrument was programmed when not in remote.

3. A trigger overrun has occurred (the instrument was triggered while processing a reading
from a previous trigger).

4. A number error has occurred (a calibration value was out of limits).

Keep in mind that you can program the instrument to assert SRQ under any of these conditions
simply by setting bit 5 in the SRQ mask (M32X). Paragraph 3.2.13 describes how to use the U1
command to obtain information on the type of error from the instrument. The U1 command is
used to clear the error bit and restore operation of SRQ on error after the error byte is read.

Programming

Notes

Examples M3X SRQ on reading overflow and data store full.

1. Note that the status byte should be read to clear the SRQ line once the instrument has
generated an SRQ. All bits in the status byte will be latched when the SRQ is generated. Bit
6 (RQS) will be cleared when the status byte is read.

2. Even with SRQ disabled, the status byte can be read to determine appropriate instrument
conditions. In this case, bits 0, 1, 3, and 4 will be continuously updated to reflect current
instrument status; however, bit 5 (the error bit) will latch and remain so until the U1 status
word (paragraph 3.2.13) is read, even if no SRQ occurs.

3. See Appendix D for an example program that demonstrates SRQ and serial polling.

M40X SRQ on reading done and on error.
M32X SRQ on error only.

3-11

IEEE-488 Programming

3.2.9 Baseline suppression (N)

Purpose To enable or disable baseline suppression.

Format N0 Disable suppression

N1 Enable suppression

Default Upon power-up, or after receiving a DCL or SDC command, suppression will be disabled (N0).

Description The baseline suppression mode allows a stored offset value to be subtracted from subsequent

readings. When the suppression mode is enabled with the appropriate command, the instrument
will internally store the baseline value with the next triggered conversion. All subsequent read­ings will be the difference between the stored baseline value and the actual signal le vel. For ex­ample, if 100mV is stored as a baseline, that value will be subtracted from the following
readings. See paragraph 2.10.2 for a complete description.

To use baseline suppression, perform the following steps:

1. Cancel baseline suppression by sending N0X if already enabled.

2. Select a range and function consistent with the expected measurement.

3. Connect the signal to be used as a baseline to the instrument input.

WARNING

The voltage present on the input terminals may be larger than
the displayed value. For example, if a 150VDC baseline is stored,
an applied voltage of +175V will result in a displayed reading of
only +25V.

4. Enable baseline suppression by sending N1X over the bus. The baseline will be stored when
the command is executed.

5. Disconnect the baseline signal from the instrument, and connect the signal to be measured
in its place. Subsequent readings will be the difference between the baseline and the applied
signal.

Programming 1. Baseline suppression reduces the dynamic range of the measurement. For example, if the

Notes stored baseline value is 100mV on the 200mV range, an input voltage of 100mV or more

would over-range the instrument e ven though v oltages up to 199.99mV are normally within
the capabilities of the 200mV range. If the instrument is in the auto-range mode, it will
move up range if necessary.

2. Setting the range lower than the stored baseline value will over-range the instrument.

3. Accurate control over when the baseline is actually stored may be achieved by placing the
instrument in a one-shot trigger mode. Once the desired baseline value is connected to the
instrument, the baseline can be stored by sending N1X.

4. Function changes cancel baseline suppress. Refer to paragraph 2.10.2 for details concerning
suppress.

3-12

Examples N1X Enable suppress.

N0X Disable suppress.

3.2.10 Data store mode (Q)

Purpose To control the data store rates and trigger mode.

Format Q0 Conversion rate (one reading every 360msec)

Default Upon power-up, or after receiving a DCL or SDC, data store is disabled (Q7).

Description The data store commands enter the data storage mode and allow you to store up to 100 readings

IEEE-488 Programming

Q1 One reading per second
Q2 One reading every 10 seconds
Q3 One reading per minute
Q4 One reading every 10 minutes
Q5 One reading per hour
Q6 Trigger mode (TRIG button)
Q7 Data store disabled

within the internal memory of the Model 6512. By entering an appropriate parameter, readings
may be stored at one of six intervals between the conv ersion rate and one reading per hour . Either
during or after the storage process, readings, including maximum and minimum values, may be
recalled by using the B command as described in paragraph 3.2.2.

Once the unit has stored all 100 readings, it will stop data storage until another Q command is
sent to enable data store once again. Note that the instrument may be programmed to generate
an SRQ when memory is full, as described in paragraph 3.2.8.

In addition to the various rates, data store can be used at a rate determined by the TRIG button
(Q6 mode). When in this mode, one reading will be stored in memory each time TRIG b utton is
operated.

Programming 1. To use data store on a one-shot basis with other trigger sources, place the instrument

Notes in the Q0 mode, and select the desired one-shot trigger mode (paragraph 3.2.12).

2. In Q0 and Q1, the storage rate may be decreased if the instrument is in auto-range and a
range change occurs.

3. See Appendix D for an example program that demonstrates data store operation.

Examples Q1X Enable data store, one per second rate.

Q7X Disable data store.

3-13

IEEE-488 Programming

3.2.11 Range (R)

Purpose To select the measurement range.

Format Volts Amps Ohms Coul. External Feedback

R0 Auto Auto Auto Auto Auto
R1 200mV 2pA 2kΩ 200pC 200mV
R2 2V 20pA 20kΩ 2nC 2V
R3 20V 200pA 200kΩ 20nC 20V
R4 200V 2nA 2MΩ 20nC 20V
R5 200V 20nA 20MΩ 20nC 20V
R6 200V 200nA 200MΩ 20nC 20V
R7 200V 2µA 2GΩ 20nC 20V
R8 200V 20µA 20GΩ 20nC 20V
R9 200V 200µA 200GΩ 20nC 20V
R10 200V 2mA 200GΩ 20nC 20V
R11 200V 20mA 200GΩ 20nC 20V
R12 Cancel auto-ranging for all functions.

Default Upon power up, or after receiving a DCL or SDC command, the instrument will be in the R0

(auto-range) mode.

Description The range command gives you control over the measurement sensitivity of the instrument. This

command, and its options, perform essentially the same functions as the front panel AUTO and
▲ and ▼ range buttons.

________________________________________________________

Programming 1. The instrument will be ready to take a reading after the range is set up when respond-

Notes ing to a range command.

2. When auto-range is disabled (R12), the instrument will stay on its present range.

Examples R12X Disable auto-range.

R3X Choose 200pA range (in amps).
R0X Enable auto-range.

3-14

3.2.12 T rigger mode (T)

Purpose T o control the source that triggers readings as well as whether the instrument operates in the con-

Format T0 Continuous Mode, Triggered by Talk

Default Upon power-up, or after the instrument receives a DCL or SDC command, the T6 (continuous

Description Triggering provides a stimulus to begin a reading conversion within the instrument. Triggering

IEEE-488 Programming

tinuous or one-shot trigger mode.

T1 One-shot Mode, Triggered by Talk
T2 Continuous Mode, Triggered by GET
T3 One-shot Mode, Triggered by GET
T4 Continuous Mode, Triggered by X
T5 One-shot Mode, Triggered by X
T6 Continuous Mode, Triggered with External Trigger
T7 One-shot Mode, Triggered with External Trigger

mode, external trigger) mode will be enabled.

may be done in two basic ways: in a continuous trigger mode, a single trigger stimulus is used
to restart a continuous series of readings. In a one-shot mode, a single reading will be processed
each time the appropriate trigger stimulus is given.

The trigger modes are paired according to the type of stimulus that is used to trigger the instru­ment. In the T0 and T1 modes, triggering is performed by addressing the Model 6512 to talk. In
the T2 and T3 modes, the IEEE-488 multiline GET command performs the trigger function. The
instrument execute (X) character provides the trigger stimulus in the T4 and T5 modes, while a
trigger pulse applied to the rear panel EXTERNAL TRIGGER INPUT, triggers the instrument
in the T6 and T7 modes.

Programming 1. A trigger stimulus will abort the present reading con version and immediately begin another .

Notes 2. The front panel TRIG button will trigger the instrument regardless of the selected trigger

mode, unless LLO is in effect.

3. Serial polling addresses the instrument to talk. This talk command will trigger the instru­ment in the T0 and T1 modes.

3-15

IEEE-488 Programming

3.2.13 Status (U)

Purpose To request status, error, and data conditions from the instrument.

Format U0 Send status word

Description The status commands allow access to information concerning instrument operating modes that

Status Word When the command sequence U0X is transmitted, the instrument will transmit the status word

U1 Send instrument error conditions
U2 Send instrument data conditions

are controlled by other device-dependent commands such as F (function) and R (range). Addi­tional parameters of the status command allow data and error conditions to be accessed.

instead of its normal data string the next time it is addressed to talk. The status word will be
transmitted only once each time the U0 command is given. To make sure that correct status is
transmitted, the status word should be requested as soon as possible after the command is trans­mitted.

The format of U0 status is shown in Figure 3-3. Note that the letters correspond to modes pro­grammed by the respective device-dependent commands. The default values in the status word
(upon power up or after a DCL or SDC command) are also shown in Figure 3-3.

3-16

IEEE-488 Programming

D

DEFAULT: 0 00 1 0 0 6 0 0 0 0 7 00 0 = :

6512 F RR C Z N T 0 B G 0 Q MM K YY <CR LF>

MODEL NUMBER PREFIX

FUNCTION
0=VOLTS
1=AMPS
2=OHMS
3=COULOMBS
4=XFDBK

RANGE

Volts Amps Ohms Coulombs XFDBK
00 = Auto Auto Auto Auto Auto
01 = 200mV 2 pA 2kΩ 200pC 200mV
02 = 2 V 20 pA 20kΩ 2nC 2 V
03 = 20 V 200 pA 200kΩ 20nC 20 V
04 = 200 V 2nA 2MΩ 20nC 20 V
05 = 200 V 20 nA 20MΩ 20nC 20 V
06 = 200 V 200 nA 200MΩ 20nC 20 V
07 = 200 V 2µA 2GΩ 20nC 20 V
08 = 200 V 20 µA 20GΩ 20nC 20 V
09 = 200 V 200 µA 200GΩ 20nC 20 V
10 = 200 V 2mA 200GΩ 20nC 20 V
11 = 200 V 20mA 200GΩ 20nC 20 V
12 = Auto off for all functions

ZERO CHECK
0 = OFF
1 = ON

ZERO CORRECT
0 = OFF
1 = ON

SUPPRESS
0 = OFF
1 = ON

TRIGGER
0 = CONTINUOUS, TALK
1 = ONE-SHOT, TALK
2 = CONTINUOUS, GET
3 = ONE-SHOT, GET
4 = CONTINUOUS, EXTERNAL
5 = ONE-SHOT, X
6 = CONTINUOUS, EXTERNAL
7 = ONE-SHOT, EXTERNAL

TERMINATOR
ASCII
= : CR LF
: = LF CR

EOI; BUS HOLD-OFF
0 = 1 EOI + HOLD-OFF
1 = NO EOI + HOLD-OFF
2 = EOI + NO HOLD-OFF
3 = NO EOI + NO HOLD-OFF

SRQ
00 = DISABLED
01 = READING OVERFLOW
02 = DATA STORE FULL
08 = READING DONE
16 = READY FOR COMMAN
32 = ERROR

DATA STORE
0 = CONVERSION RATE
1 = 1 RDG/SEC
2 = 1 RDG/10 SEC
3 = 1 RDG/MIN
4 = 1 RDG/10 MIN
5 = 1 RDG/HR
6 = TRIG BUTTON
7 = DISABLED

ALWAYS 0

DATA PREFIX
0 = PREFIX, NO SUFFIX
1 = NO PREFIX OR SUFFIX
2 = PREFIX AND SUFFIX (IF B1

READING MODE
0 = ELECTROMETER
1 = DATA STORE
2 = MAXIMUM
3 = MINIMUM

ALWAYS 0

Figure 3-3

U0 status word and default values

3-17

IEEE-488 Programming

Error The U1 command allows access to Model 6512 error conditions in a similar manner. Once the sequence

Conditions U1X is sent, the instrument will transmit the error conditions with the format shown in Figure 3-4 the

Note that all returned values except for those associated with the terminator correspond to the pro­grammed numeric values. For example, if the instrument is presently on the R3 range, the R bytes in the
status word will correspond to an ASCII 03. The returned terminator characters are derived by ORing
the actual terminator byte values with 30H. For example, a <CR> character has a decimal value of 13,
which equals 0DH. ORing this value with 30H yields 3DH, or 6110, which prints out as an ASCII equal
sign (=). This terminator conversion step is necessary to convert the standard terminators into display­able form, as they will not normally print out on a computer CRT.

next time it is addressed to talk in the normal manner. The error condition word will be sent only once
each time the U1 command is transmitted. Note that the error condition wor id actually a string of ASCII
characters representing binary bit positions. An error condition is also flagged in the status (serial poll)
byte, and the instrument can be programmed to generate an SRQ when an error condition occurs. (See
paragraph 3.2.8). Note that all bits in the error condition (U1) word and the status byte error bit will be
cleared when the U1 word is read. In addition, SRQ operation will be restored after an error condition by
reading U1.

6512 0/1 0/1 0/1 0 0/1 0/1 000 <CR LF>

MODEL

NUMBER

PREFIX

1 = IDDC
1 = IDDCO
1 = NO REMOTE
1 = TRIGGER OVERRUN
1 = NUMBER ERROR

ALWAYS

ZEROES

TERMINATOR

(DEFAULT

SHOWN)

Figure 3-4

U1 status (error condition) format

The various bits in the error condition word are described as follows:
IDDC—Set when an illegal device dependent command (IDDC) such as H1X is received (“H” is ille-

gal).
IDDCO—Set when an illegal device-dependent command option (IDDCO) such as T9X is recei ved (“9”

is illegal).
No Remote—Set when a programming command is received when REN is false.

NOTE

3-18

The complete command string will be ignored if an IDDC, IDDCO or No Remote
error occurs.

IEEE-488 Programming

Trigger Overrun—Set when a trigger is recei ved when the instrument is still processing a reading from a pre­vious trigger.

Number Error—Set when an out-of-range calibration value is received.

Data In a similar manner, the U2X sequence allows access to instrument data conditions. When this command is

Conditions transmitted, the instrument will transmit the data condition word shown in Figure 3-5 the next time it is ad-

dressed to talk. This information will be transmitted only once each time the command is received. As with
the U1 error word, the U2 word is made up of ASCII characters representing binary values. Unlike the U1
error word, howev er , the U2 data condition word will not be cleared when read; thus, instrument status in the
U2 word is always current.

6512 0/1 0 0/1 0/1 0/1 0000 <CR LF>

MODEL

NUMBER

PREFIX

1 = DATA
STORE FULL

Z (ZERO CORRECT)
0 = OFF
1 = ON

N (SUPPRESS)
0 = OFF
1 = ON

1 = TEMPORARY CALIBRATION

ALWAYS

ZEROES

TERMINATOR

(DEFAULT

VALUES

SHOWN)

Figure 3-5

U2 status (data condition) format

The various bits in the data condition word include:
Data Store Full—Set when all 100 readings have been stored in the data store memory . Cleared by requesting

a data store reading over the bus.
Z and N—Represents the same information as the corresponding zero correct (Z) and suppress (N) bytes in

the U0 status word.
T emporary Calibration—Set when new calibration parameters not yet stored in NVRAM hav e been received,

or if power-up recall of NVRAM data was in error. Cleared when NVRAM storage is performed.

Programming See Appendix D for an example program that can be used to request the status, error, and data

Note conditions words from the instrument.

Examples U0X Request status word.

U1X Request error word.

3-19

IEEE-488 Programming

3.2.14 T erminator (Y)

Purpose To program the terminator character(s) the Model 6512 adds to the end of its reading and status

Format Y<LF> <CR> <LF> <CR> (two terminator characters)

Default Upon power-up, or after a DCL or SDC command, the terminator sequence is <CR> <LF>.

Description The terminator sequence that marks the end of the instrument’s data string or status word can

Programming 1. Capital letters A through Z cannot be used as terminator characters.

Notes 2. Programming non-standard terminators may cause the controller input sequence to hang up.

Examples Y@X Program @ character as terminator.

strings.

Y<CR> <LF> <CR> <LF> (two terminator characters)
Y<ASCII> ASCII character (except A-Z)
YX No terminator

be programmed by sending the Y command followed by an appropriate ASCII character. The
default terminator sequence is the commonly used carriage return, line feed (<CR> <LF>) se­quence (<CR>=ASCII 13; <LF>=ASCII 10).

YX Disable terminator.

3.2.15 Execute (X)

Purpose To instruct the Model 6512 to execute device-dependent commands.

Format X

Description The execute command is implemented by sending an ASCII “X” over the bus. Its purpose is to

Programming Command strings sent without the execute character will be stored within an internal command

Note buffer for later e xecution. When the X character is finally transmitted, the stored commands will

Examples F0X Select DC volts function.

direct the Model 6512 to execute other device-dependent commands such as F (function) or R
(range). Usually, the e xecute character is the last byte in the command string (a number of com­mands may be grouped together into one string); however, there may be certain circumstances
where it is desirable to send a command string at one time, and then send the execute character
later on.

be executed, assuming that all commands in the previous string were valid.

X Execute previous commands.

3-20

3.2.16 Zero correct (Z)

Purpose To zero correct the instrument, nulling any internal offsets that might affect accuracy.

Format Z0 Zero correct off

Z1 Zero correct on

Default Upon power-up, or after receiving a DCL or SDC command, zero correct will be disabled (Z0).

Description The zero correct and zero check commands work together to cancel any internal offsets that

might upset accuracy. If the instrument is placed in the zero correct mode with zero check en­abled, it will store a new offset value to be used for subsequent readings. If the instrument is
zero corrected with zero check disabled, the previously stored zero value will be used instead.
Note that the specifications at the front of this manual assume that the instrument has been prop­erly zeroed.

Use the following procedure to zero the instrument:

1. With zero correct off, place the instrument in zero check by sending C1X.

2. Zero correct the instrument by sending Z1X.

3. Disable zero check by sending C0X. Readings can then be taken in the usual manner.

IEEE-488 Programming

Programming After sending a zero check command, the instrument will be ready for a new command when

Note the front end is set up.

Examples Z1X Zero correct instrument.

Z0X Disable zero correct.
C1XZ1C0X Perform complete correction.

3-21

IEEE-488 Programming

3.3 Bus connections

The Model 6512 is to be connected to the IEEE-488 bus
through a cable equipped with standard IEEE-488 connec­tors, an example of which is shown in Figure 3-6. The con­nector is designed to be stacked to allow a number of parallel
connections. Two metric screws are located on each connec­tor to ensure that connections remain secure.

Figure 3-6

IEEE-488 connector

Connect the Model 6512 to the cable as follows:

1. With the power off, line up the connector on the cable
with the connector on the rear panel of the instrument.
See Figure 3-8 for connector location.

2. Tighten the screws securely, but do not overtighten
them.

3. Add additional connectors from other instruments, as
required.

4. Make sure the other end of the cable is properly connect­ed to the controller.

NOTE

The IEEE-488 bus is limited to a maxi­mum of 15 devices, including the control­ler. Also, the maximum cable length is
limited to 20 meters, or 2 meters times the
number of devices, which ever is less.
Failure to heed these limits may result in
erratic bus operation.

A typical connecting scheme for the bus is shown in Figure
3-7. Each cable normally has the standard connector on each
end. These connectors are designed to be stacked to allow a
number of parallel connections on one instrument.

NOTE

To avoid possible damage, it is recom­mended that you stack no more than three
connectors on any one instrument.

INSTRUMENTINSTRUMENTINSTRUMENT

CONTROLLER

Figure 3-7

IEEE-488 connections

SH1

AH1T5TE0L4LE0

SR1

RL0

PP0DC1

DT1C0E1

Figure 3-8

Model 6512 rear panel IEEE-488 connector

3.4 Primary address

The Model 6512 must receive a listen command before is
will respond to addressed commands. Similarly, the unit
must receive a talk command before it will transmit its data.
The Model 6512 is shipped from the factory with a pro­grammed primary address of 27. Until you become more fa­miliar with your instrument, it is recommended that you
leave the address at this value because the e xample programs
included in Appendix D of this manual assume that address.

The primary address may be set to any value between 0 and
30 as long as address conflicts with other instruments are
avoided. Note that controllers are also given a primary ad­dress, so you must be careful not to use that address either.
Most frequently, controller addresses are set to 0 or 21, but
you should consult the controller’s instruction manual for de-

3-22

tails. Whatever primary address you choose, you must make
certain that it corresponds with the value specified as part of
the controller’s programming language.

IEEE-488 Programming

1. Press the PROGRAM SELECT button so that the fol­lowing message is displayed:

To check the present primary address, or to change to a new
one, use the following sequence:

1. Press the PROGRAM SELECT button, and note that the
following message is displayed:

IEEE 27

2. This message indicates that the IEEE address program is
selected, along with the presently programmed value (in
this case, the default value of 27 is being displayed).

3. Using one of the ADJUST buttons, scroll the displayed
address to the desired value from 0 to 30 (the display
will show special values for the talk-only mode, as de­scribed in the next paragraph).

4. Exit the program by pressing SHIFT then SELECT EX­IT. The new address is now in effect, and it will remain
programmed even if the power is turned off.

NOTE

Each device on the bus must hav e a unique
primary address. Failure to observe this
precaution will result in improper bus op­eration.

IEEE 27

2. Press the up arrow ADJUST button repeatedly until the
desired talk-only parameter (40 or 41) is shown.

3. To exit the program, press SHIFT then SELECT EXIT.
The unit is now programmed for the talk-only mode, and
it will remain programmed in this manner even if the
power is turned off.

Selecting the data output rate

The data output rate in the talk-only mode can be selected as
follows:

1. Press and hold the DATA STORE ON/OFF button until
the desired rate is displayed, as indicated below.

Displayed r Value/data output rate
_______________________________________

r=0 Conversion Rate (Every 360msec)
r=1 One reading per second
r=2 One reading every 10 seconds
r=3 One reading per minute
r=4 One reading every 10 minutes
r=5 One reading per hour
r=6 One reading each time TRIG is pressed

3.5 T alk-only mode

The Model 6512 may be placed into the talk-only mode and
be used with a listen-only device such as a printer. When in
this mode, the instrument will ignore commands given over
the bus and merely output data as requested by the listening
device. When the instrument is in the talk-only mode, the
front panel TALK LED will turn on.

The instrument can be placed in the talk-only mode by enter­ing one of the following parameters in the primary address
program:

40 Talk-only mode with prefix on data string

(Example: NDCV-1.2345E-01)

41 T alk-only mode without prefix on data string

(Example: -1.2345E-01)

To place the instrument in the talk-only mode, perform the
following steps:

2. Press the PROGRAM SELECT button until the IEEE
program message is displayed, and then release the but­ton. Select the desired IEEE-488 talk-only parameter
(40 or 41) using an ADJUST button.

3. Press SELECT EXIT to return to normal operation. The
instrument will then enter the talk-only mode and output
readings over the IEEE-488 bus at the selected interv al.

3.6 Front panel messages

The Model 6512 has a number of front panel messages asso­ciated with IEEE-488 programming. These messages are in­tended to inform you of certain conditions that occur when
sending device-dependent commands to the instrument.

The following paragraphs describe the front panel error mes­sages associated with IEEE-488 programming.

3-23

IEEE-488 Programming

3.6.1 Bus error

A bus error will occur if the instrument receives a de vice-de­pendent command when it is not in remote, or if an illegal de­vice-dependent command (IDDC) or illegal device­dependent command option (IDDCO) is sent to the instru­ment. Under these conditions, the complete command string
will be rejected, and the following message will be dis­played:

b Err

In addition, the error bit and pertinent bits in the U1 word
will be set (paragraphs 3.2.13 and 3.2.16), and the instrument
can be programmed to generate an SRQ under these condi­tions (paragraph 3.2.13).

No Remote error

A no remote error can occur when a command is sent to the
instrument when the REN line is false. Note that the state of
REN is only tested when the X character is received.

3.6.3 T rigger overrun error

A trigger overrun error occurs when the instrument receives
a trigger while it is still processing a reading from a previous
trigger. Note that only the overrun triggers are ignored and
will have no effect on the instrument except to generate the
message below . When a trigger o verrun occurs, the following
front panel message will be displayed for approximately one
second:

t Err

3.7 Bus data transmission times

A primary consideration is the length of time it takes to ob­tain a reading once the instrument is triggered to make a con­version. The length of time will vary somewhat depending
on the selected function and trigger mode. Table 3-4 gives
typical times.

Table 3-4

Trigger to reading-ready times

IDDC error

An IDDC error can occur when an invalid command such as
H1X is transmitted (this command is invalid because the in­strument has no command associated with that letter).

IDDCO error

Similarly, an IDDCO error occurs when an invalid option is
sent with a valid command. For example, the command T9X
has an invalid option because instrument has no such trigger
mode.

3.6.2 Number error

A number error occurs when an out of range calibration com­mand (A) value is sent to the instrument. Under these condi­tions, the instrument will display the following error
message:

n Err

The command string will be accepted, but the calibration val­ue will remain unchanged.

Configuration

2V, 20V, 200V
200mV

Time
(msec)

365
780

Error (1% of
step input)

0.01

0.1

20nA, 200nA, 20µA,

200µA, 20mA
20pA, 200pA
2nA, 2µA, 2mA
2pA
2nC, 20nC
200pC
20kΩ-200GΩ
2kΩ

NOTES:

1. Conditions: Input is on range, and times may depend on controller

used.

2. Preamp settling time (to 12%) is 2 seconds on preamp ranges (2, 20,

200pA), and must be taken into account by the user.

3. Volt time/error also applies to external feedback.

365
365
780
780
365
780
365
780

0.01

2

55

0.1

2

25

0.01

0.1

0.01

0.1

3-24

4

Applications

4.1 Introduction

Applications for the Model 6512 are many and varied and
will depend on your needs. Basically, the Model 6512 can be
used to make many of the same measurements in the range
of ordinary DMMs; however, special characteristics such as
high input resistance and high sensitivity give the instrument
much better capabilities than those of the ordinary DMM.

For example, the typical input resistance for an ordinary
DMM is on the order of 10M Ω . In contrast, the Model 6512
has an input resistance of greater than 200T Ω (2 × 10
The Model 6512 can detect currents as low as 0.1fA

-16

(10

A), while a typical DMM might be limited to current

measurements in the µA range.

In this section, we will discuss some possible applications
for the Model 6512 Electrometer. K eep in mind that these ex­amples are only representative of what is possible with this
highly sophisticated instrument, and by no means exhaust
the possible uses for the unit.

14

Ω ).

An example of a situation requiring low current measure­ment is shown in Figure 4-1. In this example, the gate leak­age current of a JFET is to be measured. Although the de vice
manufacturer may specify the current value, it is often desir­able to verify the specification for a particular sample of the
device. Also, the specified leakage current might be at a
higher voltage than required. For example, the specified
leakage current might be 1nA with an applied voltage of
25V, while that figure might be much less at an operating val­ue of 10V.

This application requires the use of an external voltage source
such as the Model 230 shown in the figure. The voltage source
can be programmed to the desired value or values, and the
leakage current could be measured for each voltage. In this
manner, leakage current characterization studies could be per­formed with a minimum of effort.

As shown in Figure 4-1, a shielded test fixture should be used
to keep the measurement quiet and stable. A good quality
low-noise triax cable should be used to connect the current
input to the instrument.

4.2 Low-level leakage current
measurements

Many devices exhibit lo w-lev el leakage currents that may re­quire measurement. Typically, such leakage currents might
lie in the nA (10
range. The Model 6512 is an ideal instrument for such cur­rent measurements because it can detect currents as low as

0.1fA.

-9

A), pA (10

-12

A) or even the fA (10

-15

A)

Forward and reverse diode currents could be measured in a sim­ilar manner. The forward leakage current (measured with the
voltage source set to less than 0.6V) can be measured using the
Model 6512 without regard to input voltage burden. High-capac­itance diodes such as zener devices will present no problem,
since the Model 6512 is unaffected by stray capacitance up to

0.01µF .

4-1

Applications

TRIAX CABLE

FET

I

L

UNDER

TEST

SHIELDED

TEST FIXTURE

OFF
VΩ, GUARD

COMMON

HI

INPUT

LO

ON

MODEL 6512

OUTPUT

Figure 4-1

Leakage current measurement

MODEL 230 VOLTAGE SOURCE

TEST FIXTURE

I

L

HI

LO

A. CONNECTIONS

6512 PREAMP

HI
LO

GND

COMMON

OUTPUT

B. EQUIVALENT CIRCUIT

A/D CONVERTER

230 VOLTAGE

SOURCE

4-2

Applications

4.3 Diode characterization

When the Model 6512 is placed in the ohms mode, constant
current values between 1nA and 100µA are av ailable at the IN­PUT jack high and low terminals, as shown in Table 4-1. (Input
high sources the current.) These currents can be used to plot the
I-V (current-voltage) characteristics over a substantial range.

Table 4-1

Diode currents and voltages

Diode

Range

2k Ω , 20k Ω
200k Ω
2M Ω
20M Ω
200M Ω
2G Ω , 20G Ω , 200G Ω

current Diode voltage (V)*

-6

-9

-6

-6

) (R)

) (R)

-9

-9

) (R)

) (R)

) (R)

) (R)

100µA
10µA
1µA
100nA
10nA
1nA

V = (100 × 10
V = (10 × 10
V = (1 × 10
V = (100 × 10
V = (10 × 10
V = (1 × 10

*R = displayed resistance.

Figure 4-2 shows the basic circuit configuration for using the
Model 6512 in this manner. A decade current, I, is forced
through the diode under test. The current will develop a for­ward voltage drop, V

, across the diode. The voltage across the

F

diode can be calculated by multiplying the displayed resis­tance by the test current (see Table 4-1). For example, assume
that a resistance reading of 50k Ω is measured with the instru­ment on the 200k Ω range. The voltage across the diode is:

V

= 10µA × 50k Ω = 0.5V

F

Figure 4-3 shows several examples for typical diodes. The
curves were drawn from data obtained in the manner described.

WARNING

Up to 300V may be present between the
high and low terminals in ohms.

Figure 4-1

Diode characterization

INPUT

ON

OFF

V, Ω GUARD

SHIELDED FIXTURE

TRIAX CABLE

6512 SET TO OHMS

I

HI

V

F

LO

A. CONNECTIONS

HI

LO

GND

B. EQUIVALENT CIRCUIT

6512 PREAMP

S

HI

LO

SHIELDED

TEST FIXTURE

V

I

DIODE

V

F

UNDER

TEST

TO A/D

4-3

Applications

100µA

10µA

DIODE

CURRENT

(I)

1µA

100nA

10nA

1nA

1N914

1N4148

1N4006

1N765

0.1 0.2 0.3 0.4 0.5
V

F

0.6

Figure 4-2

Diode curves

4.4 Capacitor leakage measurements

An important parameter associated with capacitors is their
leakage currents. Once the leakage current is known, the in­sulation resistance can be calculated. Ideally, a capacitor
should have no leakage current, and thus infinite leakage re­sistance. However, capacitors, like all practical devices, are
not ideal, so these parameters can become important, espe­cially to circuit design and component engineers. The
amount of leakage current in a given capacitor depends on its
dielectric as well as the applied voltage. Ceramic dielectric
capacitors typically have leakage currents in the nA to pA
range, while polystyrene and polyester dielectric devices ex­hibit much lower leakage currents–generally in the fA range.
(These values are for test voltages in the 100V range).

The basic configuration for this test is shown in Figure 4-4.
A Model 230 Voltage Source is used to impress a voltage
across the capacitor, C. The resulting leakage current is then
measured by the Model 6512.

SHIELD

R

HI

230

VOLTAGE

SOURCE

LO

RECOMMENDED R VALUES: < 100pF-10nF 1MΩ

I

L

C

HI

I

M

LO

10nA-1µF 100kΩ

6512

ELECTROMETER

IN AMPS

Figure 4-3

Capacitor leakage tests

The resistor R is necessary to limit current to a safe value in case
the capacitor is shorted, and it also helps to reduce noise. Typi­cally a value of about 1M Ω should be used, although that value
can be decreased for larger capacitor values. However, values
under 10k Ω are not recommended. (Refer to paragraph 2.13.8.)

At the start of the test, the Model 6512 should be placed in
the amps mode and on the 20mA range. The Model 230 Volt­age Source is then programmed to the desired voltage, and its
output is turned on. Once the required soak time has passed,
the Model 6512 can be placed on the proper current range to
make the current measurement. (The soak time is the period
necessary to fully charge the capacitor; typically 10RC.)
Once the test is completed, the voltage source should be
turned off to allow the capacitor to discharge.

The leakage current can be directly read from the Model
6512 display during the test procedure. If the leakage resis­tance value is required instead, the value can be calculated
from the source voltage and the measured current.

This basic procedure could be used to test a number of capac­itors on an automated basis. A test fixture that holds a num­ber of capacitors could be constructed, and a Keithley Model
7001 or Model 7002 Switch System, equipped with appro­priate scanner cards, could be used to select among the vari­ous devices to be tested. For a higher degree of automation,
the switch system, voltage source, and Model 6512 could be
controlled from a computer via the IEEE-488 bus. In this
way , measurements that would otherwise be tedious and time
consuming could be conducted on a more routine basis.

4-4

C

2nC

100V

-------------- - 20pF==

Applications

4.5 Capacitance measurement

The coulombs function of the Model 6512 provides a quick
and easy method of measuring capacitance values of capaci­tors, cables, and connectors. It is especially useful in cases of
cables and connectors because of the very small values of
charge that can be measured.

The basic method involv es using a Model 230 voltage source
to apply a step voltage across the capacitor, as shown in Fig­ure 4-5. Since charge is to be measured, the Model 6512
should be in the coulombs function to make the measure­ment. Just prior to turning on the voltage source, zero check
should be disabled and the charge suppressed. Then, turn on
the voltage source and note the final charge value.

SHIELD

2

1

C

C

Q

M

10nF-1µF 100kΩ

∆Q

-------- -=

∆V

HI

ELECTROMETER

IN COULOMBS

LO

6512

R

HI

230

VOLTAGE

SOURCE

LO

RECOMMENDED R VALUES: < 100pF-10nF 1MΩ

V

V

STEP
VOLTAGE

Figure 4-4

Capacitor measurement

The capacitance can then be computed as follows:

As an example of the above procedure, assume that an un­known capacitor is to be measured. If the step voltage is 100V,
and a Q value of 2nC is obtained, the capacitance value is:

4.6 Insulation resistance measurements

At the moderate impedance levels of many circuits, insula­tion resistance is seldom a consideration, as it is generally
many orders of magnitude above the highest impedance en­countered in the remainder of the circuit. At very high im­pedance levels, however, insulation resistance can be a
consideration, since it can lower effective circuit impedance
considerably. Since typical insulation resistances run in the
range of 10
ment range of ordinary instruments. The high resistance
measurement range of the Model 6512, however, gives it ca­pabilities to measure such high resistances.

4.6.1 Unguarded resistance measurements

A typical test configuration for making unguarded insulation
resistance measurements is shown in Figure 4-6. The con­stant-current method is used, and insulation resistances up to
200G Ω can be measured using the Model 6512 ohms func­tion. As the term implies, the test current through the un­known resistance is kept constant, while the voltage
developed across the test resistance will, of course, depend
on the value of the insulation resistance. The Model 6512
measures the generated voltage and calculates the resistance
value accordingly . The lo w compliance voltage of the Model
6512 (<2V on 2G Ω range and lower, except <300V during
overload) keeps error due to voltage coefficient small.

10

16

- 10

Ω , their values lie above the measure-

where:

∆ Q = Q
∆ V = V

(final charge) - Q

2

(step voltage) - V

2

(initial charge, assumed to be 0)

1

(initial voltage, assumed to be 0)

1

4-5

Applications

INPUT

OFF

ON

VΩ, GUARD

6512 SET TO OHMS, VΩ, GUARD OFF

TEST FIXTURE

HI

I

R

X

LO

TRIAX CABLE

A. CONNECTIONS

6512 PREAMP

I

HI
LO

GND

HI

SHIELDED TEST FIXTURE

R

V

RX =

LO

I

V

I

R

X

INSULATION

RESISTANCE

TO A/D

B. EQUIVALENT CIRCUIT

Figure 4-5

Insulation resistance measurement (unguarded)

4.6.2 Guarded resistance measurements

For resistance measurements above 10
er than three feet, guarded measurements are recommended,
as shown in Figure 4-7. In this case, the rear panel V, Ω
GUARD switch is used to apply a guard signal to the inner
shield of the connecting triax cable. The guard is carried
through to the inner shield of the test fixture. The inner shield
must be insulated from the outer shield, which is a safety
shield. Incidentally, a shielded fixture is recommended for
both unguarded and guarded resistance measurements above

7

10

Ω if stable readings are to be expected (in the unguarded

mode, the shield should be connected to input low).

8

Ω , or for cables long-

Since the inner shield carries the guard signal, the COM ter­minal acts as circuit LO.

4.6.3 V/I resistance measurements with external
voltage source

With the constant-current method discussed abov e, the Mod­el 6512 can make measurements as high as 200G Ω . Howev­er, the insulation resistance of such materials as polyethylene
may lie above this range. By using the Model 6512 along
with an external voltage source to make resistance measure­ments in the constant-voltage mode, measurement range can
be extended up to 10
constant-voltage method will be faster.

16

Ω . Also, for a given resistance, the

4-6

INPUT

OFF

ON

VΩ, GUARD

COM

TRIAX CABLE

SAFETY
SHIELD

R

X

UNKNOWN

RESISTANCE

SIGNAL
GUARD

Applications

6512 SET TO OHMS, VΩ, GUARD ON

SAFETY SHIELD

I

R

X

SIGNAL
GUARD

Figure 4-6

Insulation resistance measurement (guarded)

WARNING: SAFETY SHIELD RECOMMENDED FOR GUARDED
RESISTANCE MEASUREMENTS ABOVE 30GΩ. UP
TO 300V MAY BE PRESENT ON GUARD.

A. CONNECTIONS

COM

LO

6512 PREAMP

I

HI
GUARD

GND

B. EQUIVALENT CIRCUIT

R

V

A/D CONVERTER

A typical configuration for using the Model 6512 to make
V/I resistance measurements is shown in Figure 4-8. Here,
an external Model 230 Voltage Source is used to force a cur­rent, I, through the unknown resistance, R. The insulation re­sistance can then be calculated as follows:

V

R

----=

I

I is the current through the resistance as measured by the
Model 6512, and V is the programmed Model 230 voltage.
For example, assume that the applied voltage is 100V, and
the measured current is 1pA. The resistance is calculated as
follows:

100V

-------------- -

R

1pA

1014Ω==

Since the Model 230 voltage source can be adjusted over a
wide range of values (up to ±101V), this configuration can be
used for voltage coefficient studies, as described later in this
section.

Note that COM is connected to input LO through 100 Ω and
appears in series with the resistor under test. This resistance
is below the resolution of the instrument for measurements
above 2M Ω .

In addition to the measurement of insulation resistances, this
basic method can be used to measure unwanted leakage re­sistances. For example, leakage resistance between PC board
traces and connectors can be made with either of the two
methods described above, depending on the resistance values
involved.

4-7

Applications

COMMON

INPUT

ON

OFF

V, Ω GUARD

OUTPUT

MODEL 6512

TRIAX CABLE

HI

I

R

X

LO

UNKNOWN
RESISTANCE

SHIELDED

TEST FIXTURE

MODEL 230 VOLTAGE SOURCE

TEST FIXTURE

HI

R

X

LO

B. EQUIVALENT CIRCUIT

Figure 4-7

Insulation resistance measurement using external voltage source

A. CONNECTIONS

6512 PREAMP

HI
LO

GND

COMMON

OUTPUT

A/D CONVERTER

230 VOLTAGE

SOURCE

4-8

Applications

ERROR

100MΩ

100MΩ

200TΩ+

----------------------------------------------

100×=

% % = 0.00005% error

100 R1R

1

–()

R1V∆()

-----------------------------------

Voltage Coefficient (%/V) =

4.7 High-impedance voltmeter

The input resistance of the Model 6512 in the volts mode is
greater than 200TΩ. Because of this high value, the Model
6512 can be used to make voltage measurements in high-im­pedance circuits with a minimum of loading effects on the
circuit.

Consider the situation where a circuit designer must measure
the gate-to-source voltage of a precision JFET amplifier that
has a gate impedance of 100MΩ. Further assume that the re­quired accuracy of this measurement is 1%.

The setup for this measurement is shown in Figure 4-9. The
gate-source voltage is represented by VGS, while the effec­tive gate impedance is represented as RS. The input resis­tance of the voltmeter is given as RIN.

METER

HI

R

IN

A. MEASUREMENT
CONFIGURATION

LO

V

GS

Figure 4-8

Measuring high-impedance gate-source voltage

The percent error due to voltmeter loading in this circuit can
be given as:

%%

ERROR

METER

HI

R

IN

LO

B. EQUIVALENT CIRCUIT

R

S

------------------------

RSR

+

100×=

IN

R

S

100MΩ

V

GS

Such a large error would not be tolerable in this case because
of the 1% accuracy requirement. However, since the Model
6512 has an input resistance of 200TΩ, its error in this exam­ple would be:

Note that this error term is so small as to be insignificant as it
would be dominated by the instruments’s accurac y specification.

Thus, the input impedance of the Model 6512 would be more
than adequate for this situation, because the error due to
meter loading is substantially better than the required 1%
value stated earlier . In addition, the 4½ digit resolution of the
instrument allows the designer sufficient precision to make
use of the high input impedance.

4.8 Voltage coefficients of

high-megohm resistors

High-megohm resistors (above 109Ω) often exhibit a change
in resistance with applied voltage. This resistance change is
characterized as the voltage coefficient. The Model 6512 can
be used in conjunction with a Model 230 Voltage Source to
determine the voltage coefficient of such resistors.

The basic configuration for making voltage coefficient mea­surements is shown in Figure 4-10. The voltage, VS, is ap­plied to the resistor under test by the voltage source of the
instrument. The current is measured by the electrometer in­put of the Model 6512. The resulting current can then be
used to calculate the resistance.

Two resistance readings at two different voltage values will
be required to calculate the voltage coefficient. The voltage
coefficient in %/V can then be calculated as follows:

Suppose, for example, a typical DMM with a 10MΩ input
resistance were used to make this measurement. The error
because of meter loading would be:

100MΩ

ERROR

% % = 91% error

----------------------------------------------

100MΩ 10MΩ+

100×=

Even if a DMM with an input resistance of 1GΩ (109Ω) were
used, the error would still be:

100MΩ

ERROR

% % = 9.1% error

-----------------------------------------

100MΩ 1

100×=

GΩ+

where:
R1 is the resistance with the first applied voltage.
R2 is the resistance with the second applied voltage.
∆V is the difference between the two applied voltages.

4-9

Applications

As an example, assume that the following values are obtained:

R1 = 1.01 × 1010Ω
R2 = 1 × 1010Ω
∆V = 5V

TRIAX CABLE

INPUT

ON

OFF

V, Ω GUARD

MODEL 6512

COMMON

OUTPUT

The resulting voltage coefficient is:

8

×()

Voltage Coefficient (%/V) =

100 1 10

--------------------------------- -

110105()×

Note that the voltage coefficient of a particular device may
apply only across the selected voltage range, and may, in
fact, vary with different voltage increments in the same ap­proximate range.

HI

I

HIGH

R

X

MEGOHM

RESISTOR

LO

SHIELDED

TEST FIXTURE

MODEL 230 VOLTAGE SOURCE

TEST FIXTURE

HIGH

MEGOHM

RESISTOR

R

X

Figure 4-9

Configuration for voltage coefficient studies

A. CONNECTIONS

6512 PREAMP

HI

LO

V

HI
LO

GND

COMMON

S

OUTPUT

B. EQUIVALENT CIRCUIT

A/D CONVERTER

230 VOLTAGE

SOURCE

4-10

Applications

LO

TOP VIEW

SIDE
VIEW

INNER

CYLINDER

BNC
CONNECTOR

INSULATORS

(TEFLON OR CERAMIC)

OUTER

CYLINDER

4.9 Static charge detection

Electrostatic charge is a deficiency or excess of electrons on
an ungrounded surface. Such charges are usually generated
on poor conductors of electricity such as plastics, synthetic
fibers, and paper during handling or processing of these ma­terials. Once these charges accumulate, they do not dissipate
readily because of the excellent insulating characteristics of
the materials involved.

Static charge build-up can be a problem with integrated cir­cuits, especially with those of the CMOS variety. While
these devices, which operate at high impedance levels, often
have static protection built in, it is best to properly protect
them during transit or storage. For that reason, such ICs are
usually shipped and stored in anti-static tubes.

A primary consideration, then, is the degree of static protec­tion afforded by the anti-static tube. A comparison among
various tubes can be set up to test the variations in charge
build-up as a particular IC slides the length of the tube. The
charge value will, of course, be measured by the Model 6512
being operated in the coulombs function.

To perform the test, connect the Model 6512 to the Faraday
cup using a suitable shielded cable, such as Model 4801
BNC cable. A Model 7078-TRX-BNC triax-to- BNC adapter
will be required to make the connection. W ith the instrument
in the coulombs mode, place a typical IC in the tube to be
tested; allow it to slide the full length of the tube and fall into
the Faraday cup. The amount of charge built up during the
test will then be registered on the Model 6512.

The test can be repeated with other tubes, as required. In order
for the test to be valid, all tubes should be the same length, and
the same IC should be used in every case. The tube that gener ­ates the smallest static charge as seen on the electrometer is the
one with the best anti-static characteristics. The amount of
charge seen during this test will depend on many factors, in­cluding the type of tube material, tube length, the IC used, as
well as the relative humidity. Typical values might be in the

0.5-1nC range for a good anti-static tube, while one without
anti-static protection might generate 10 times that amount.

To perform this test, a test fixture called a Faraday cup will
be necessary. Such a fixture can be easily constructed from
two cans, as shown in Figure 4-11. For example, the outer
can could be a one-gallon paint can, while the inner cylinder
could be one of slightly smaller diameter, such as a quart
paint can. The two cans must be insulated from one another.
Although the type of insulator is not all that critical, ceramic
or Teflon insulators can be used.

For conv enience, a BNC connector could be mounted on the
outside can. The outer, or shield connection will, of course,
be connected to the outer can, while the inner conductor
should be connected to the inner can.

Figure 4-10

Faraday cup construction

4-11

5

Performance V erification

5.1 Introduction

The procedures outlined in this section may be used to verify
that the instrument is operating within the limits stated in the
specifications at the front of this manual. Performance veri­fication may be performed when the instrument is first re­ceived to ensure that no damage or misadjustment has
occurred during shipment. Verification may also be per­formed whenever there is a question of instrument accuracy,
or following calibration, if desired.

WARNING

The procedures in this section are in­tended only for qualified service person­nel. Some of the procedures may expose
you to hazardous voltages. Do not per­form these tests unless you are qualified
to do so.

NOTE

If the instrument is still under warranty
(less than one year from the date of ship­ment), and its performance falls outside
the specified range, contact your Keithley
representative or the factory to determine
the correct course of action.

5.3 Initial conditions

The Model 6512 must be turned on and allowed to warm up
for at least two hours before beginning the verification pro­cedures. If the instrument has been subjected to extremes of
temperature (outside the range specified in paragraph 5.2),
additional time should be allowed for internal temperatures
to reach normal operating temperature. Typically, it takes
one additional hour to stabilize a unit that is 10°C (18°F) out­side the specified temperature range.

NOTE

While rated accuracy is achieved after the
two-hour warm up period, input bias cur­rent may require additional time to settle
to its optimum level. Allow two hours for
input bias current to settle to less than
10fA and eight hours to less than 5fA. It is
preferable in sensitive applications to
leave the unit on continuously.

5.4 Recommended test equipment

The following equipment is needed to verify all measuring
functions of the Model 6512:

• Keithley Model 263 Calibrator/Source (source volts,
ohms, amps, and coulombs).

5.2 Environmental conditions

All measurements should be made at 18-28°C (65-82°F) and
at less than 70% relative humidity unless otherwise noted.

• Fluke 343A DC Calibrator (190V; ±0.002%).

• 2-slot triax cable (supplied with Model 263).

5-1

Performance Verification

• Keithley Model 6171 3-slot male to 2-lug female triax
adapter.

• Single banana cable.

• Keithley Model CAP-31 protective cap/shield for IN­PUT connector.

5.5 V erification procedures

The following paragraphs contain procedures for verifying in­strument accuracy with each of the four measuring functions:
volts, ohms, amps and coulombs using a Model 263 Calibra­tor/Source. Using the Model 263 to verify Model 6512 simpli­fies the procedure and eliminates tedious characterization
procedures and the need to build a special test fixture.

These procedures are intended for use only by qualified per­sonnel using accurate and reliable test equipment. If the in­strument is out of specifications, refer to Section 7 for
calibration procedures, unless the unit is still under warranty .

WARNING

The maximum common-mode voltage
(voltage between input low and chassis
ground) is 500V. Exceeding this value
may cause a breakdown in insulation,
creating a shock hazard. Some of the
procedures in this section may expose
you to dangerous voltages. Use standard
safety precautions when such danger­ous voltages are encountered.

high volts or ohms. Place the V, Ω
GUARD switch in the OFF position un­less otherwise noted.

5.5.1 Input current verification

Perform input current verification as follows:

NOTE

The following procedure must be performed
at an ambient temperature of 23°C ±1°C.

1. Disconnect all cables from the Model 6512 input.

2. Place the CAP-31 input cap on the INPUT connector.

3. Select the amps function, 2pA range, enable zero check,
and then enable zero correct.

4. Connect a jumper between the rear panel COM and
chassis ground terminals.

5. Disable zero check, and allow one minute for the read­ing to stabilize.

6. Verify that the reading is 50 counts or less (within
50fA). Enable zero check.

7. Remove the jumper connected between the COM and
chassis ground terminals.

5.5.2 Amps verification

Connect the Model 6512 to the Model 263, as shown in Fig­ure 5-1, and perform amps verification as follows:

5-2

CAUTION

The maximum voltage between the high
and low input terminals is 250V (10 sec­onds maximum on the mA ranges). In­strument damage may occur if this
value is exceeded.

NOTE

Verify performance in the order listed: in­put current, amps, coulombs, volts, and
ohms. Input current may remain high for
several minutes following measurement of

1. Enable Model 6512 zero check, and select the 20mA
range. Do not use auto-range.

2. Check that the display reads 0.000 ±1 count. If not, en­able zero correct.

3. Using the AMPS (acti v e) current source mode, program
the Model 263 to output 19.0000mA to the Model 6512.

4. Disable zero check, and verify that the reading on the
Model 6512 is within the limits in Table 5-1.

5. Using Table 5-1 as a guide, repeat steps 1 through 4 for
the 2mA through 2nA current ranges.

6. Using the AMPS V/R (passive) current source mode of
the Model 263, repeat steps 1 through 4 for the 200pA
through 2pA ranges.

L
C

Performance Verification

* Reading limits shown include Model 6512 and Model 263 accuracy specifications.

5.5.3 Coulombs verification

Connect the Model 6512 to the Model 263 as shown in Figure 5-1, and perform coulombs verification as follows:

4. Program the Model 263 to output 1.90000nC, and press

OUTPUT

2-S

TRIAX

OPERATE to source charge to the Model 6512.

5. Verify that the Model 6512 reading is between 1.8943
and 1.9057nC.

MODEL 263

Figure 5-1

Connections for amps and coulombs verification

Table 5-1

Limits for amps verification

6512
range

20mA
2mA
200µA
20µA
2µA
200nA
20nA
2nA
200pA
20pA
2pA

263
function

AMPS
AMPS
AMPS
AMPS
AMPS
AMPS
AMPS
AMPS
AMPS V/R
AMPS V/R
AMPS V/R

263
output

19.000mA

1.90000mA

190.000µA

19.0000µA

1.90000µA

190.000nA

19.0000nA

1.90000nA

190.000pA

19.0000pA

1.90000pA

Allowable reading
(1 Year, 18°C-28°C)

18.978 - 19.022mA

1.8973 - 1.9027mA

189.76 - 190.24µA

18.976 - 19.024µA

1.8973 - 1.9027µA

189.59 - 190.41nA

18.965 - 19.035nA

1.8962 - 1.9038nA

187.45 - 192.55pA

18.770 - 19.230pA

1.8811 - 1.9189pA

1. Using the COUL (active) charge source of the Model
263, select the 2nC range.

2. Place the Model 263 in the coulombs function, and per­form zero correction by enabling zero check and zero
correct in that order.

3. Disable zero check on the Model 6512.

5.5.4 Volts verification

NOTE

Current and charge verification must be
performed before volts verification.

Connect the Model 6512 and 190V calibration source to the
Model 263, as shown in Figure 5-2, and perform volts verifi­cation as follows:

1. On the Model 6512, enable zero check, and select the
200mV range.

2. Check to see that the display reads 000.00 ±1 count. If
not, enable zero correct.

3. Program the Model 263 to output 190.000mV.

4. Disable zero check, and verify that the reading on the
Model 6512 is within the limits listed in Table 5-2.

5. Using Table 5-2 as a guide, repeat steps 1 through 4 for
the 2V and 20V ranges.

6. Set the Model 6512 to the 200V range.

7. Set the external calibration source to output 190.000V to
the Model 263.

8. Source 190.000V to the Model 6512 by pressing SHIFT
VOLTS on the Model 263.

9. Verify that the reading on the Model 6512 is within the
limits listed in the table.

5-3

Performance Verification

10. Enable zero check on the Model 263, and turn off the external calibration (190V) source.

5-4

Performance Verification

DUAL BANANA CABLE

DC CALIBRATOR

EXTERNAL CALIBRATOR SOURCE

Figure 5-2

Connections for volts verification

HI

CHASSIS

LO

Table 5-2

Limits for volts verification

6512
range

200mV
2V
20V
200V

263
output

190.000mV

1.90000V

19.0000V

190.000V

EXT INPUT

COMMON

MODEL 263

2-SLOT
TRIAX
CABLE

Allowable reading
(1 year, 18°-28°C)

189.91 to 190.09mV

1.8993 to 1.9007V

18.993 to 19.007V

189.86 to 190.14V

6171 3-SLOT MALE TO

2-LUG FEMALE TRIAX

V, Ω GUARD

OFF

ADAPTER

MODEL 6512

* The 200mV, 2V , and 20V ranges allowable readings include Model 263 error. The 200V range reading is based solely on Model 6512 accuracy specifica­tions.

5.5.5 Ohms verification

Connect the Model 6512 to the Model 263, as shown in Figure 5-3, and perform ohms verification as follows:

NOTE

Charge and current verification must be performed before resistance verification.

1. Set the Model 6512 to the 2k Ω range.

2. Zero correct the Model 6512 by enabling zero check and zero correct in that order.

3. Set the Model 263 to the 1k Ω range, and while in OPERATE, press ZERO to source zero ohms to the Model 6512.

4. Release zero check on the Model 6512, and allow the reading to settle.

5. On the Model 6512, press SUPPRESS to cancel offset and test lead resistance.

6. On the Model 263, source the 1k Ω resistor to the Model 6512. The actual value of the output resistance is displayed on the
Model 263.

7. Record the Model 263 reading in Table 5-3.

8. Calculate the Model 6512 reading limit using the formula in the table.

9. Verify that the reading on the Model 6512 is within the limits calculated in step 8.

10. Referring to Table 5-3, repeat the basic procedure in steps 3 through 9 for the 20k Ω range.

5-5

Performance Verification

11. For the remaining Model 6512 ranges, repeat steps 6
through 9 by sourcing the appropriate resistance to the
electrometer. Note that guard must be enabled on both
the Models 6512 and 263 when verifying the G Ω rang­es. Also, note that COM of the Model 6512 must be con­nected to COMMON of the Model 263 (see Figure 5-3).

5-6

Figure 5-3

COMMON

MODEL 263

BA

OUTPUT

Connections for ohms verification

Table 5-3

Limits for ohms verification

Performance Verification

Calculated limit

6512
range

2k Ω
20k Ω
200k Ω
2M Ω
20M Ω
200M Ω
2G Ω
20G Ω
200G Ω

* Includes Model 263 error.

263 output
(nominal)

1k Ω
10k Ω
100k Ω
1M Ω
10M Ω
100M Ω
1G Ω
10G Ω
100G Ω

6512 & 263
guard

Off
Off
Off
Off
Off
Off
On
On
On

Equipment
error* Limit

(____ × 0.16%) + 0.0004k Ω = ____
(____ × 0.13%) + 0.001k Ω = ____
(____ × 0.23%) + 0.01k Ω

= ____
(____ × 0.225%) + 0.0001M Ω = ____
(____ × 0.2125%) + 0.001M Ω = ____
(____ × 0.23%) + 0.01M Ω = ____
(____ × 1.4%) + 0.0001G Ω = ____
(____ × 1.275%) + 0.001G Ω = ____
(____ × 1.1%) + 0.01G Ω = ____

Allowable 6512 reading
(1 year, 18°-28°C)263 reading

263 Reading ± Limit
263 Reading ± Limit
263 Reading ± Limit
263 Reading ± Limit
263 Reading ± Limit
263 Reading ± Limit
263 Reading ± Limit
263 Reading ± Limit
263 Reading ± Limit

5-7

Performance Verification

C

R

5.5.6 Input impedance verification

Perform this test to verify that the input impedance of the
Model 6512 is greater than 200T Ω :

1. Connect the DC calibrator, Model 263, and the Model
6512 together as shown in Figure 5-4.

2. Place the Model 6512 in the volts function, select the
20V range, and enable ZERO CHECK. Verify that the
display shows 0.000V ±1 count. If not, enable ZERO
CORRECT.

3. Enable GUARD on both the Models 6512 and 263.

4. On the Model 263, select the 200G Ω range, and press
ZERO to source zero ohms. Make sure the Model 263 is
in OPERATE.

5. Set the DC calibrator to output 19.000V.

6. On the Model 6512, disable ZERO CHECK, and note
the reading.

7. Enable ZERO CHECK on the Model 6512, and press
ZERO on the Model 263 to select the 100G Ω

8. Disable ZERO CHECK on the Model 6512. After wait­ing a few seconds for settling, note the reading on the
Model 263.

9. Compare the reading obtained in step 8 with that noted
in step 6. The two readings should be within 10 counts
(10mV) of one another.

resistor.

SHORTING LINK

COMMON

BANANA CABLE

DISCONNECTED

OUTPUT

MODEL 263

(GUARD ON)

Figure 5-4

Connections for input impedance verification

T

D

5-8

6

Theory of Operation

6.1 Introduction

This section contains an overall functional description of the
Model 6512 in block diagram form, as well as details of the
various sections of the instrument. Information concerning
the electrometer section, mother board circuitry, IEEE-488
interface, power supplies, and display circuitry is included.

Information is arranged to provide a description of each of
the functional blocks within the instrument. Many of these
descriptions include simplified schematics and block dia­grams. Detailed schematic diagrams and component layout
drawings for the various circuit boards are located at the end
of Section 8.

6.2 Overall functional description

A simplified block diagram of the Model 6512 is shown in
Figure 6-1. The instrument may be divided into three discrete
sections: analog, digital, and power supplies. The analog and
digital sections are electrically isolated from one another by
using opto-isolators for control and communications. Sepa­rate power supplies for the various analog and digital sec­tions ensure proper isolation. Because of these isolation
techniques, the analog low connection may be floated up to
±500V above chassis ground, while digital common may be
floated up to ±30V above ground.

The analog section consists of the input stage, output stage,
ranging amplifier, A/D converter, and feedback and switch-

ing elements. The input stage is a proprietary FET amplifier
designed for high input impedance (200T Ω ) and low input
offset current (less than 5fA). The output stage provides fur­ther amplification, thus allowing the preamp output to go as
high as ±210V, depending on the selected range and func­tion. Further control of the input and output stages is provid­ed by the feedback and switching elements, which set gain
and transfer function according to the selected range and
measuring function. In addition, zero check and zero correct
provide a conv enient means to zero the instrument, allowing
cancellation of internal offsets.

The ranging amplifier conditions the output stage signal into
a 0-2V signal for the A/D converter. The A/D converter uses
both charge balance and single-slope conv ersion techniques.

The heart of the digital section is the 146805E2 CMOS pro­cessor that supervises the entire operation of the instrument.
Additional digital circuits include the display made up of a 4
1/2 digit mantissa and a 2-digit alpha or numeric exponent, the
IEEE-488 interface, and the front panel switch matrix. The
switch matrix decodes front panel switch closure information
that controls instrument operation from the front panel.

Because of the diversity of circuitry within the Model 6512,
a number of power supply voltages are required. The analog
section requires ±5V (input stage) and ±210V and ±24V sup­plies (output stage). Additional supplies include a separate
+5V and -9.1V supply for A/D circuits, and a separate +5V
supply for digital circuitry.

6-1

Theory of Operation

I

N

HI

I

N

LO

GND

FEEDBACK
ELEMENTS,
SWITCHES

INPUT
STAGE

OUTPUT

STAGE

RANGING
AMPLIFIER

ANALOG

CONVERTER

A/D

ANALOG DIGITAL

±5 ±210 ±24 +5 +5

-9.1

POWER SUPPLY

Figure 6-1

Overall block diagram

6.3 Input preamplifier

The input preamplifier provides the high input impedance and
high output voltage capability necessary for the volts and ohms
functions, and the low input impedance and high current output
capability needed for the amps and coulombs functions.

A simplified block diagram of the input preamplifier is
shown in Figure 6-2. The circuit is essentially made up of
three sections: an input stage, which provides the necessary
input impedance functions, a gain stage, which provides the
needed amplification, and an output stage, which supplies
the required voltage or current drive capability. Additional
feedback and switching elements configure the amplifier ac­cording to the selected range and measuring function.

µP DISPLAY

SIGNAL

INPUT

FRONT

PANEL

BUTTONS

INPUT
STAGE
(Q308)

DIGITAL

IEEE-488

INTERFACE

FEEDBACK

AND

SWITCHING

GAIN
STAGE
(U309)

OUTPUT

STAGE
(Q301,

Q302,-Q307)

Figure 6-2

Basic configuration of electrometer preamplifier

OUTPUT TO

RANGING
AMPLIFIER

6-2

Figure 6-3

Electrometer preamplifier configuration

Theory of Operation

The exact configuration of the input preamplifier will depend
on the measuring function. Figure 6-3 shows circuit config­uration for the four measuring functions. In the volts func­tion, the circuit is set up as a high-input impedance (2 ×

14

10

Ω ), unity gain, non-inverting buffer amplifier. In the

ohms function, a bootstrapped reference is placed in series
with a range resistor (R
through the measured resistance (R

) that drives a constant current

F

). The reference has a

X

value of 10V, 1V or 0.1V, depending on the selected range.
The voltage developed across the unkno wn resistance is pro­portional to its value.

In the amps and coulombs modes, the circuit is configured as
a feedback type current-to-voltage converter. In the amps
mode, the feedback element is a resistor, with the value de­pendent on the selected range. In the coulombs mode, the
feedback element is a capacitor.

V

= V

OUT

IN

6.3.1 Input stage

A simplified schematic of the input stage is shown in Figure
6-4. The primary purpose of this stage is to provide low leak­age characteristics of the input preamplifier.

Stage operation centers around a dual JFET , Q308. Resistors
R314, R342, R351 and R352 provide a means to balance the
circuit with help of jumper W303. Depending on circuit off­set, jumper W303 should be placed in one of three positions:
A, B, or C.

Signal input is applied to the gate of the left JFET section
through R334. The characteristics of the right JFET section
remain constant since its V
cause of the variation in the characteristics of Q308A, the
current through R335 varies, developing a proportional out­put signal that is applied to the next stage.

R

F

voltage stays constant. Be-

GS

A. VOLTS

C. AMPS

R

=

R

V

VR = 10V, 1V

or 0.1V

V

OUT

IF =

X

x V

R

F

-Q

=

OUT

V

IN

C

F

OUT

V

R

R

F

I

F

V

OUT

V

IN

R

F

I

IN

V

OUT

= -IIN R

V

OUT

F

R

X

B. OHMS

Q

IN

D. COULOMBS

V

OUT

CF = 1000pF

6-3