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