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Service Manual
Model 181
Digital Nanovoltmeter

01981, Keithley Instruments, Inc.
October 1981, Cleveland, Ohio, U.S.A.
Document Number 30816
Model 181 Service Manual Addendum

INTRODUCTION
This addendum to the Model 181 Service Manual is being provided in order to supply you with
the latest information in the least possible time. Please incorporate these changes into the manual
before servicing the Model 181.

Model 262 Low Thermal Voltage Divider

This addendum concerns availability of the Keithley Model 262 Low Thermal Voltage Divider,
which is recommended for verifying accuracy and calibrating the Model 181 mV ranges. The
Model 262 is a precision low thermal divider with divider ratios of 101:1, lO?l, lO":l, and lO?l. A
low thermal male-to-male output cable is included with the Model 262. Note that Model 181 veri-
fication and calibration procedures that use the Model 262 are included in the Model 262 Instruc-
tion Manual.

Section 2, page 2-2:

Include the Model 262 Low Thermal Voltage Divider in the list of available accessories.

Table 3-1, page 3-1:

Replace Item B in the table with the Model 262

Table 5-1, page 5-l:

Replace Items B and F with the Model 262

I
TABLE OF CONTENTS

SECTION TITLE PAGE
1 Generallnformation ....................................
l-l Introduction ............................................
l-2 Warranty Information ....................................
1-3 ManualAddenda ........................................
1-4 Safety Symbols and Terms. ...............................

2 Accessories ............................................ .......
2-l General ................................................
2-2 Model 1463 Low-Thermal Connection Kit ....................
2-3 Model 1484 Refill Kit .....................................
2-4 Model 1485 Female Connector .............................
2-5 Model1486MaleConnector ...............................
2-6 Model 1488Shorting Plug .................................
2-7 Model 1503 Low-Thermal Solder and Flux ...................
2-6 Model1506lnputCable ...................................
2-9 Model1507lnputCable ...................................
z-10 Model 1615Maintenance Kit ...............................
2-11 Model1019 Universal Rack Mounting Kit ....................

3 Performance Verification ............................... .........
3-l General ........................................... ....
3-2 Recommended Test Equipment ............................
3-3 Environmental Conditions .................................
3-4 Performance Verification Procedure ........................
3-5 Initial Conditions .........................................
3-6 DC Voltage Accuracy Check 12V to lO@S/) ..................
3-7 DC Voltage Accuracy Check (2mV to 2OOmV) ................
3-6 HiResolutionCheck ......................................
3-9 Damping Function Check ....................
3-10 Filter Function Check ........................
3.11 NoiseCheck ...............................

4 Theory of Operation ........ .............. .,.,4-l
4-l General .... ............................ ~~~ 4-l
4-2 Power Supply ..............................
4-6 IEEE Standard Interface ......................
4-7 Digital Section .............................
4-8 Microcomputer Memory ..................... .........
4-9 AID Converter Control ......................
4-10 AID Operation .............................
4-11 Single Slope Phase ......................... .........
4-12 Serial/Parallel Conversion, Isolator Control ..... .........
4-13 Clock Circuit .. ... ..................... .........
4-14 Display .................................... .........
4-15 D/A Converter .............................
4.16 Nanovolt Preamp ...........................
4-17 Low Noise Design ..........................
4-16 Linearity and Gain ..........................
4-19 Feedback Elements .........................
4-20 Offs&Null .................................
4-21 NMRR Filter and Suffer ......................
4-22 Drift Correction ................. .. .... ...........
4-23 Input Protection .. ......................
4-24 ,nputMuxPumpout .........................
4-25 FET Multiplexing ........................... ...........
4-26 InputMux .................................
4-27 AID Converter .............................
4-28 Timing ....................................
4-29 MuxFETs .................................
4-30 Transconductance Amplifier lGAMPl ..........
4-31 integrator .................................. ...........
4-32 Reference Supply ...........................
TABLE OF CONTENTS
(CONTINUEDI

SECTION TITLE PAGE

5 Maintenance ......................................................................................... 5-l
5-l General ........................................................................................... ...5- 1
5-2 RecommendedTestEquipment..........................................................................5- 1
5~3 VoltageDivider........................................................................................6 -1
5-4 EnvironmentalConditons ............................................................................. ..5 -2
5-5 CalibrationProcedure ................................................................................ ..5~ 2
5-6 Installation of the Calibration Cover ...................................................................... .5-2
5-7 AIDCalibration(ZVRangel .......................................................................... ...5 -2
5-6 lOMegAdjustment.....................................................................................5~ 3
5-9 20VRangeAdjustment.................................................................................5 -3
5-10 200VRangeAdjustment................................................................................5~ 3
5-11 lOOOVRangeAdjustmant...............................................................................5- 3
5-12 DIACalibration ..................................................................................... ..5~ 3
5-13 NanovolrPreampCalibration .......................................................................... ..5~ 3
5-14 Troubleshooting.......................................................................................5- 4
5.15 Special Handling of Static Sensitive Devices .............................................................. .5-4
5-16 Line Power ........................................................................................... 5-4
5-17 AIDConverterandDisplay..............................................................................5 -7
5-18 DisplayBoard ...................................................................................... ...5 -9
5.19 Nanovolt Preamp Noise Troubleshooting ................................................................. .5-9
5-20 Preamp NoiseTroubleshooting Checks .................................................................. .5-9
5-21 ChecksofPreampZero.................................................................................5- 9
5-22 BufferCheck.........................................................................................5-1 2
5-23 InputCurrentCheck ................................................................................ ...5-1 2
5-24 BootstrapCheck.. .................................................................................. ..5~12
5-25 LowNoiseSupplyCheck...............................................................................5~1 2
5~26 CurrentBiasCheck.. ................................................................................ ..5~12
5-27 InputFETReplacemenr ............................................................................ ,.,.5-l 2
5-26 NanovoltPreampTroubleshooting.......................................................................5-1 3
5-29 DC Volts Attenuator Troubleshooting ................................................................... ,513
5-30 lEEEBusCircuitTroubleshooting........................................................................5-1 3
5-31 DIACircuitTroubleshooting............................................................................5-1 3
5-32 SwitchlnputSectionTroubleshooting....................................................................5-1 3
5-33 DigitalTroubleshooting .............................................................................. ..5~15

6 Replaceable Parts. ....... ......... ....... ......... .6-l
6-1 General .................. ......... ....... ......... ..6-1
6-2 Ordering Information ...... ......... ,.6-l
6-3 Factory Service ........... ,.6-l
6-4 Component Layouts ....... ....... ......... ..6-1
6-5 Schematics ............... ......... ......... ..6-1

AppendixA............,,..,..,,.,....................,...............................................A-l

ii
LIST OF ILLUSTRATIONS

FIGURE TITLE PAGE
1-l Front Panel ......... ......... . . ..l-2
1-2 Rear Panel ......... ......... ..~.1.2

2-l Model 1483 ~2~1
2-2 Model 1485 2~1
2-3 Model 1486 2-l
2-4 Model 1488 2~1
2-5 Model 1503 2~1
2-6 Model 1506 2~1
2-7 Model 1597 2~1
2-8 Model 1815 2-2
2-9 Model 1019 ~2~2

3-l DCVPerformanceCheck.........................................,............~~.~~,~~..~..~~,~..,~ ~3.1
3-2A 2mVand20mVPerformanceCheck ..~..~...............~...... 3~2
3-28 200 Millivolt Performance Check ~. ~.

4-l Latch Enable ............................ ~4~2
4-2 AID Control Logic ....................... ~4-2
4-3 Charge Balance Timing ................... .4~3
4-4 Single Slope Timing. ..................... ~, ,4-3
4-5 Charge Balance-Single Slope Phase ......... ~4~4
4-6 Simplified Nanovolr Preamp Schematic ...... 4~5
4-7 NVPA Showing Filter Et Drift Correction. ..... ~4.7
4-8 Preamp AID Timing. ...................... 4~8
4-9 V Range A/D Timing ...................... 4-9
4-11 Transconductance Amplifier. .............. ,4-9
4-10 Simplified A/D Diagram .................. 4-10
4-12 Integrator ............................... 4~11
4.13 Reference Supply ........................ 4-11

5-l ShieldedEnclosure..................................................~....~~~ ....... ~~,~~ ...... 5.2
5-2 ConnectorAssembly......................................................................~......~.~.~ ~~~5~2
5-3 MaletoMaleConnector ............................................................................ ..~..~~~5- 2
5-4 DividerConnection...............................................................................~..~~~~~~.5~ 2
5-5 PreampCalibration....................................................~......~.~....~....~~~~~~.~~~~~~~.~ 5.3
5-6 DCCalibrator,Kelvin-VarleyDividerandW.. ............... ............. ............ ~.~,~~~~~ ......... ~5~4
5-7 Model1815CalibrationCover..................................................~ ............. ,.,,.~,,~ 5.5
5-8 480kHzClockl~sec/cm2V/cm..............................~ .............................. ,~, 59
5-9 2.4kHzClock.2~eclcm2Vicm .............................................................. .,,, 5-9
5-10 lnputDisablelOmseclcm2V/cm...................................................................~ .. ,5-9
5-11 lnputDisable50mseclcm2V/cm ...................................................................... ~, 5~9
5-12 ATlOlOutputl0msec/cm2Vlcm..............................................................~ ... ,.~ ,, 5~9
5-13 StrobePulsesl0mseclcm2Vlcm..................................................~.......~~~ ........ ~~~.5~9
5-14 AT103OutputlOmseclcm2Vlcm...........................................................~..~..~..~~ ~~~5~10
5-15 AT1040utput50msec/cm2Vlcm....................................................................~......5~1 0
5-16 V-FOutputl0msec/cm2Vlcm.. .......................... .. ... .. ...... ................. ~~~5~10
5-17 lntegratorOutput10mseclcm.5Vlcm.......................................................~........~~..~..5~l 0
5-18 AID Output Buffer Output lOOmsec/cm5V/cm ........................... ............. .. .. ..... .. ,5-10
5-19 NoisyQ41310pVlcm50ms/cm .............................. .... ................ ............... ,.,,5-11
5-20 Quiet041310~VV/cmWms/cm ...................... ..... .......... ....... .................. ,5-11
5-21 NoisyU4O1lOpV/cmWms/cm.. ............................... ............. ....................... 512
5-22 0uietlJ40110~V/cm50ms/cm ................................................................... 5-12

6-l Nanwolt Preamp PC-526, Component Location Diagram No. 30377D. Rev. C .......... .. ..... 6~15
6-2 Analog Board PC-529, Component Location Diagram No. 30503D. Rev. J ... .. ........... ..... .... 6~16
6-3 Display Board PC-530, Component Location Diagram No. 30532D, Rev. E ................ .. ..... .... ... ~6~17
6-4 Digital Board PC-531, Component Location Diagram No. 30576D. Rev. J (sheet 1 of 21 ..................... ~6.18
6-4 Digital Board PC-531, Component Location Diagram No. 30676D. Rev. J (sheet 2 of 21 ......... .... .... ~6~19
6-5 NanovoltPreampPC-526,SchematicDiagramNo.30586D,Rev.A.. ....................... ~~,.~ ..... ~6~21
6-6 Analog Board PC-529, Schematic Diagram No. 30685D. Rev. C ........................... .. .. ....... 6~23
6-7 DisplayBoardPC-530.SchematicDiagramNo.30584D.Rev.C.. .................. .............. ~~,.~ ~, ,6~25
6-8 Digital Board PC-531, Schematic Diagram No. 30583D. Rev. F (sheet 1 of 21.. .......... .. .. ......... 6-27
6-8 Digital Board PC-531, Schematic Diagram NO. 30583D. Rev F (sheet 2 of 21 ............... ..... ........ ~6~29

iii
I TABLE
LIST OF TABLES

TITLE PAGE

3~1 Recommended Test Equipment for Performance Verification ...... 3-1
3-2 DC Voltage Performance Check (2V to 1OOOVl.. .............. .. .3~1
3-3 DC Voltage Performance Check (2mV to 200mVl ................ .3~2

4~1 Microcomputer Memory Map .............................. .. . ...4-2
4~2 Gain Resistors .... .................. ~, .. ................. .4~6
4~3 Gain Resistors for Volts .............. .. .................. .4~6
4-4 MuxFETs .................................................. . ...4-9

5~1 Recommended Test Equipment ............................... ......................... .,.,5-l
5~2 Voltage Divider Parts List ................................ ... . ...5-1
5~3 Power Supply Checks ................................. ... .5-6
5-4 AIDConverter ..... ....................................... .......................... .5-7
5~5 Display.. ......................................... ....... .5-a
5-6 Preamp Troubleshooting .................... ~, ............... .5~13
5~7 DCV Attenuator Troubleshooting ........................... .5~13

Replaceable Parts List PC-531 Schematic30593D ........ ...... .6~1
Replaceable Parts List PC-529 Schematic30585D ... ......... .6~6
Replaceable Parts List PC-526 Schematic 30586D ......... .6-10
Replaceable Mechanical Parts ................... ............. .6-12
Cross Reference of Manufacturers. .......................... ,613
I MODEL 181 GENERAL INFORMATION

Section 1. General Information

l-l. INTRODUCTION,
The Keithley Model 181 is a 5% and 6% digit DC voltmeter
with resolution to lOnV, The Model 181 is a unique DC
voltmeter in the respect that it combines microprocessor
technology for full programmability with a new concept in
nanovolt front ends, It provides highly accurate, stable and
low noise readings from 1OnV to 1CCQVDC on 7 voltage
ranges. The 2V through the 1CCOV ranges utilize the 5.way
binding posts. The 2mV through the 2CQmV ranges utilize the
special low thermal input connector.
The service manual contains the necessary information for
calibrating and maintaining the Model 181. This information is
provided in various sections throughout the manual. These
sections are listed as Performance Verification, Theory of
Operation and Maintenance/Calibration. Along with this
section, General Information, this manual also includes the
Accessories, a Parts List, and the Schematic Diagrams,

CMRR: 16OdS on mV ranqes. 140dS an V ranges; at DC and line fre
quency 60 or 60Hrl. ,lkR unbalance)

IEEE-488 BUS IMPLEMENTATION: Accuracy: f 10.15% of displayed reading + lm"i~
Multiline Commands: LXX. LLO. SDC. GET. The Constent: 400ms.
Unlline Commands: IFC, REN, EOI, SRD. ATN. Level: t 2V full scale on all ranges: x1 or x,000 gain
PROGRAMMASLE PARAMETERS: ISOLATION: Input LO 10 Outpur LO or power line ground' ,400" peak. 5
Front Panel Controls: Range. Filter. Zero, Dampin% Hi Resolution. x lOW.Hz. greater than ,090 paralleled by ,soJ"F
Internal Parsmeters: SRCI Resoonse. Triswsr
-- Modes. oats WARMUP: 1 hour IO rated SCCUE.CYwhen propedy zeroed,
Tetminators. ENWRONMENTAL LIMITS: OperatinS: WC 10 35