Keithley 261 Service manual

Model 261

Picoampe:re Source

Instruction Manual

Contains Operating and Servicing Information

WARRANTY

Keithley Instruments, Inc. warrants this product to be free from defects in material and workmanship for a period of 1 yew 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 bc defective.

To exercise this warranty, write or call your local Keithley representative, or contact Keithley 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.

LIMITATION OF WARRANTY

This warranty does not apply to delccts 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 * 216-248-0400 -Fax: 216-248-6168 * http:llwww.kcithley.com

Model 261 Picoampere Source

Instruction Manual

0 1983, Keithley Instruments, Inc.

Test Instrumentation Croup

Cleveland, Ohio, U.S.A.

Third Printing

Document Number: 29046 Rev. C.

Model 261 Picoampere Source

OUTPUT: lPl4A (10-1lA full range) to 1.1 x 10-4A, positive or negative, in eight decade

ranges.

AC&RACY: Exclusive of input drop consideration:

ACCURACY WITH

10.00 SET-I-ING

RANGE

SETTING SPAN, AMPERE

10-7 to KY5

lti 10-s to1cF7

1w9 10-9 to10-8 w-10 lo-l0 to 10-9 lo-11 lo-12 lo-12 to lo-11 M.7 % il.6 % lo-12 lo-14 to 10-12

‘All accuracies are f the percentage given, a.01 x range switch setting. LONG-TERM STABILITY: Will operate within stated specifications for three months after

calibration. After three months add 0.15% per month to 10-g through 10-12A range setting accuracy specifications.

TEMPERATURE COEFFICIENT: ffl.l%/“C from 15°C to 30°C on 10-7 to 10-5A range settings.

Approximately O.l5%/“C on lo-12 to 10-gA range settings. Exact values for these ranges

supplied with instrument. WARM-UP TIME: 1 hour. LINE REGULATION: 0.01% for 10% change in line voltage. SOURCE VOLTAGE: 0 to 11V in O.OlV’steps. RESOLUTION: 3 significant figures from lO-12A to 1.1 x 10-4A. RANGE RESISTORS: 105 to 10120 in decade steps; *5%. RANGE RESISTOR ACCURACY: Value with power on, given on certificate.

~5 %:108 to 101252

lt0.1 %:107 to 106 n

~0.02%:105~. OUTPUT ISOLATION: Low to ground: >109n shunted by O.OOl@ CERTIFICATION: A Calibration Certificate is furnished including range resistor values,

thermal coefficients, temperature and date of calibration. Certification traceable to the

National Institute of Standards and Technology is also available. POWER: 10%125V or 210-250V (switch selected), 50-6QHz,6W. DIMENSIONS, WEIGHT: 155mm high x 225mm wide x 300mm deep (6.25 in. x 9 in. x 12 in.).

Net weight 4.lkg (9 lbs.). ACCESSORY SUPPLIED: Model 2611 Test Cable.

10-7 to 1.1 x lrv4

l&11 to lo-10 ti.6 % a.3 %

UOV SOURCE OTHER THAN

VOLTAGE) 10.00*

iO.25%

?a.5 % MO.6 % ffl.8 % fo.6 % il.1 %

WORST-CASE

WITH SETTING

Revised 1116189

f0.25%

M.5 %

32.0 %

Paragraph

TABLE OF CONTENTS

Title

Page

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.6

1.9

2.1

2.2

2.3

2.4

2.5

2.6

2.6.1

2.6.2

2.7

2.6

2.6.1

2.6.2

2.9

2.9.1

2.9.2

2.10

2.10.1

2.10.2

2.11

2.12

2.13

3.1

3.2

3.3

3.4

4.1

4.2

4.3

4.4

4.5

4.5.1

4.5.2

4.5.3

4.6

5.1

5.2

5.3

5.3.1

5.3.2

5.3.3

6.1

6.2

6.3

6.4

SECTION 1 GENERAL INFORMATION Introduction

Model261Features.. .....................................................................

WarrantyInformation .......................................................................

ManualAddenda.. .........................................................................

SafetyTerms ..............................................................................

Unpackingar~dlnspection ...................................................................

Using the Model 261 Instruction Manual.

...

Speclflcatlon:r. Optional Accessories

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......................................................................... ....

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SECTION 2 ~OPERATION

Introduction ....................

Front Panel Controls and Terminals Rear Panel Controls and Terminals

PreparationForUse .........................................................................

BasicOperatingProcedure ...................................................................

AccuracyCollsidarations ....................................................................

Input Voltage Drop (Voltage Burden1

Proper Multiplier Settings DutputConnactions Input Voltage Drop for Fsedback and Shunt-Type Picoammeters

Feedback-‘TypePicoammeter ...............................................................

Shunt-TypNs Picoammeter Compensation for Input Voltage Drop of Feedback~Type Picoammetsr

Calculating Voltage Drop Error.,

CompensatingEquation Compensation Drop for Input Voltage Drop of Shunt~Type High-Impedance

CalculatingtheError ........................................................................

CompenstitingEquations ..................................................................

Compensation for Input Voltage Drop of Shunt-Type Low Impedance Picoammeters Range Resistor Voltage Coefficient Compensation for Temperature Coefficient

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Picoammetsrs

............

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SECTION 3 APPLICATIONS

Introduction CurrentSuppression..

GaivanometricMeasurements ................................................................

0hmmeterC;llibration .......................................................................

...............................................................................

....................

.................................................

SECTION 4 PERFORMANCE VERIFICATION AND CALIBRATION

Introduction ...............................................................................

EnvironmsntelConditions ....................................................................

RecommendedTestEquipment............................................................~ ..

InitialConditions ...........................................................................

VerificationProcedure .......................................................................

TopCoverRemoval .......................................................................

Range Resistor Verification Multiplier Switch Setting Verification

Calibration .................................................................................

................................................................

........................................................

SECTION 6 THEORY OF OPERATION

Introduction

Block Diagram

CircuitDescription ..........................................................................

VoltageSupply ...........................................................................

RangeResistors ..........................................................................

Kelvin-Varley Voltage Divider,

..................................................... .........................

.......................................... ..................................

..............................................................

SECTION 6 MAINTENANCE

Introduction ...............................................................................

ServicingSchedule .........................................................................

TopCaverRt!moval .........................................................................

PalfSReplact!me”t ..........................................................................

1-l

l-1 l-l l-1 1~1 1~1 l-1

.1-l

1~2

2-l 2-1 2-1 2~1 2-1 2-3 2-3 2-3 2-3 2-4

2-4 2-4 2-5

2-5 2-5

2-5

2-6

3-l

3-1

3-l

3-2

4-l

4-1

4-l

4-1 4-2 4-2

5-l 5-1 5-1 5-l 5-l 5-2

6-l 6-1 6-l 6-1

TABLE OF CONTENTS (CONT.)

6.6

6.5.1 Troubleshooting Equipment

6.52 CommonFeults.. ........................................................................

6.6.3 TroubleshootingGuidelines..

6.6

Troubleshooting.

Fuse Replacement ..........................................................................

...........................................................................

................................................................

..............................................................

SECTION 7 REPLACEABLE PARTS

7.1

7.2

7.3

7.4

7.5 Schematic Diagram and Component Layouts ...................................................

Introduction

Parts List ..................................................................................

Ordering Parts .............................................................................

Factory Service.

...............................................................................

............................................................................

7-1 7-l

LIST OF FIGURES

Figure Title Page

2-l

2-2

2-3 2-4 2-5 2-6 3-1

3-2 3-3 4-l

5-l 7-l

7-2 7-3 7-4

7-5 7-6

7-7 7-a

FrontPenelControlsandTerminals

Rear Panel Controls and Terminals ...........................................................

ConnectingtheMode1261Source

High Resistance Paths in Output Connections ..................................................

Simplified Circuit of Feed-Sack Tvpe Picoammeter .............................................

Tvpical Shunt Type Picoemmeter ............................................................

Current Suppression Circuit Connections .....................................................

GalvanometricMeasurements..

Guarded Circuit for Ohmmeter Calibration .....................................................

Top Chaws

BlockDiagram Voltage Supplv Board. Component Location Calibr&tion’f&d, C

Calibr&tion’f&rd, Component Location Drawing, D&g. No. 30483C AM AMPERESRangeSwitch.Sl02

10t2Kl Resistor Asssl

10t2Kl Resistor Assembly IRlOa). X.01 Multiplk X.OlMultiplisrSwitch.Sl03.. X.OlMultiplierSwitch.Sl04.. Xl Multiplier Switch, S105.

Model 261 Schematic Diagram. Dwg. No. la323E ..............................................

View

...........................................................................

............................................................................

...........................................................

.............................................................

Drawina

............................................................. 7-5

.............................................................

..............................................................

.............................................................. 7-6

.................................................................

...........................................

..............................

2-2 2-2 2-3 2-4 2-4 2-5 3-l 3-1 3-2 4-2 51 7-l 7-2

7-5 7-6

7-6 7-7

61 6-l 6-l 6-2 62

7-1 7-l 7-1

LIST OF TABLES

Table 2-l

2-2 41 4-2 4-3 4-4 4-5 61 6-2 7-l

7-2

Current Output Examples Causes of Discrepancy in Model 261 Output Current and Correction Index Recommended Equipment for Verification and Calibration Range Resistor Verification Range Resistor Accuracy

Voltage For Multiplier Switch Settings

RangeCalibration

Troubleshooting Guide

FuseTypes

Model261PartsList ........................................................................

Mechanical Parts

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Title Page

2-3

...........................

........................................

2-6 4-l 4-2 42 43 4-3 82 62 7-3

7-5

SECTION 1

GENERAL INFORMATION

1.1 INTRODUCTION

The Model 261 Picoampere Source is a secondary standard for use in calibrating picoammeters and electrometers. The output of the Model 261 can be adjusted between 10-‘4A and

1.1 x 10.4A in eight decade ranges. The source has 3 digit

resolution for current outputs b#etween 10~12 and 10~4A. The current output of the Mlodel 261 is derived from a

variable voltage source and a selectable range resistance. The voltage supply is highly ragulatled for stability and uses highquality components throughoul for reliability. The low side of the output can be floated to avoid possible ground loop problems.

1.2 MODEL 261 FEATURES Important Model 261 features iixlude:

1. 11V Internal Source. The internal voltage source can be set to a value between 0 and 1’lV in O.OlV increments. This permits accurate calibration of picoammeters and electrometers with input voltagt! drops up to 1OmV without calibration correction.

2. Stable High-Meg Resistance Standards. The highmegohm resistance standards are selected for maximum stability. Individual temperature coefficients and resistance values are supplied with each instrument.

3. Line Operation. The Model 261 is line operated from either

105.125V or 210.250V power sources. Voltage selection is easily accomplished with the rear panel line voltage selection switch.

4. Eight Current Ranges. Eight current ranges are user selectable with a single front panel control.

5. Shielded Output Connector. The UHF output connector on the front panel is Teflon@ insulated to minimize possible leakage problems.

6. Floating Low Connection. The low side of the output connection can be floated to miinimize the effects of ground IOOPS.

1.3 WARRANTY INFORMATION

Warranty information may be found inside the front cover of this manual. If warranty service is required, contact the Keithley representative in your area or the factory to determine the correct course of action. Keithley Instruments, Inc., maintains service facilities in the United States, West Germany, Great Britain, the Netherlands, Switzerland and Austria. Information concerning the application, operation, or setvice of your instrument may be directed to the applications engineer at any of these locations. Addresses can be found inside the front cover of this manual.

1.4 MANUAL ADDENDA Changes pertaining to the instrument that occur after the

printing of this manual will be found in an addendum included with this manual. Be sure to note any changes before attempting to operate or service the instrument.

1.5 SAFETY TERMS

The WARNING heading used in this manual explains dangers that could result in personal injury or death. The CAUTION heading used in this manual explains hazards that could damage the instrument.

1.6 UNPACKING AND INSPECTION

The Model 261 was carefully inspected before shipment. Upon receiving the unit, carefully unpack all items from the shipping canon and check for any damage that might have occurred during transit. Report any damage to the shipping agent at once. Save the original packing material in case reshipment becomes necessary. Contact the nearest Keithley representative or the factory if the unit fails to function properly.

The following items are included in every Model 261 shipment:

1. Model 261 Picoampere Source

2. Model 261 Instruction Manual

3. Model 2611 Test Lead Cable

4. Additional accessories as ordered.

1.7 USING THE MODEL 261 INSTRUCTION MANUAL

This manual is intended to familiarize the operator with the operating controls and features of the Model 261 Picoampere

Source. Also included is information on calibration, maintenance and spare parts. The manual is divided into the

following sections:

1. Sections 2 and 3 contains pertinent operating information,

including applications and possible pitfalls to avoid when using the source.

2. Section 4 covers performance verification and calibration procedures.

3. A complete description of operating theory is contained in Section 5.

4. Troubleshooting and maintenance procedures are covered in Section 6.

5. Parts ordering information can be found in Section 7.

1.8 SPECIFICATIONS

A complete list of Model 261 specifications can be found immediately preceding this section.

1-l

1.9 OPTIONAL ACCESSORIES The Model 4003A Rack Mounting Kit allows the Model 261

to be conveniently mounted in a standard 19 inch rack. The overall dimensions of the Model 4003A kit is 5% inches high

by 19 inches deep. Two different covers are provided for use with either 10 inch deep or 13 inch deep instruments. Contact your nearest Keithley representative or the factory for information on ordering the Model 4003A.

1-2

SECTION 2

OPERATION

2.1 INTRODUCTION This section contains information necessary to set up the

Model 261 and operate the unit. Also included are compensating equations necessary when using the source with measuring instruments with higlh input voltage drops.

2.2 FRONT PANEL CONTROLS AND TERMINALS Figure 2-l shows the front panel of the Model 261, The func-

tions of the various controls and terminals are described below.

POLARITY Switch. The POLARITY switch has four posi-

tions which are used to control power, turn the instrument output off and on, and set the polarity of the source output. In the AC OFF position, the IModel 261 power is disconnected. In the OFF position, power is applied to the instrument, but the source is disconnected from the output jack. When the POLARITY switch iri in this position, the output resistance of the source is equal to the reciprocal of the AMPERES switch setting, regwlless of the multiplier settings in use. The + and - positions s,elect positive or negative current output values.

Range (AMPERES) Switch. The range switch selects one of eight decade current ranges between 10~5 and 10.‘*A. The actual output is affected by the positions of the three multiplier switches. When the multiplier switches are in the 1 .OO positions, the range switch indicates the actual output current.

Voltage Switch. The voltage switch sets the source for

105.125V or 210.250V operation. A screwdriver can be used to set the switch to the desired position.

FUSE. By unscrewing the fuse holder cap, the fuse is accessable. See Section 6 for fuse replacement procedures.

Power Cord. The 3wire cord is permanently attached to the rear panel. The opposite end of the power cord has a standard 3-prong plug attached. This plug should be used only with grounded outlets.

COM end GND terminals. The COM terminal is connected to the low side of the source output. The GND terminal is connected to chassis ground, which is connected to the power line ground wire in the 3.wire power cord. Normally, the GND and COM terminals are connected together with a shorting link, which is provided. The link can be removed to float the low side from ground.

2.4 PREPARATION FOR USE Before operating the Model 261, perform the following steps:

1. Set the line voltage selection switch to the appropriate setting depending on the local power line voltage. Check to see that the appropriate fuse is installed.

CAUTION Operating the source on en incorrect line voltage may damage the unit. possibly voiding the warranty.

Multiplier Switches. The mull:iplier switches control the internal voltage source of the instrument. These switches provide multiplying values that determine the actual output of the source. To determine the current output, multiply the current range by the multiplier s,etting. The multiplier switches control the output through the use of a 3-decade KelvinVarley divider. The Xl and X.01 switches each have 11 positions between 0 and 10; the X:1 switch has 10 positions between 0 and 9. The illuminated decimal point between Xl and X.1 switches also serves as a pilot lamp.

OUTPUT Receptacle. The OUTPUT receptacle is a Teflon@ insulated UHF type connector. The low (threaded) side floats from ground unless the COM and GND terminals on the rear panel are connected together. A chassis ground terminal (GI is located next to the OUTPUT connector.

2.3 REAR PANEL CONTROLS AND TERMINALS Figure 2-2 shows the rear panel of the source. The purpose of

the various controls and terminals is described below.

2. Sat the Model 261 front panel controls as follows: POLARITY Switch: AC OFF Range Switch: Desired current range Multiplier Switches: 10.00

3. Plug in the source to AC power and rotate the POLARITY

switch to the OFF position. The decimal point pilot light should come on, indicating the unit is receiving power. If

not, disconnect the instrument from the power line and

check to see that the fuse is good.

2.5 BASIC OPERATING PROCEDURE Operation of the Model 261 is fairly straight forward. Once

the unit is connected to the measuring instrument, the operator need only select the appropriate range and multiplier switch settings and select the output current polarity. The basic operating procedure is as follows:

1. Plug in the instrument to the appropriate power source and rotate the POLARITY switch to the OFF position. The pilot light should indicate that power is applied to the source. Allow a one hour warm-up period for rated accuracy.

2-1

Figure 2-l. Front Panel Controls and Terminals

-II

GROUND AND

COMMON TERMINALS

III

III-

---

III

VOLTAGE SWTCH FVSE

LINE CORD

2-2

Figure 2-2. Rear Panel Controls and Terminals

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