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Operator's Manual
Model 181
Digital Nanovoltmeter
01982. Keithley Instruments, Inc
Cleveland, Ohio, U.S.A.
Document Number 32421
181 l NANOVOLTMETER
IEEE-488 BUS IMPLEMENTATION
ADDRESS MODES: TALK ONLY and ADDRESSABLE.
MULTlLlNE COMMANDS: DCL, LLCJ,xc, GET.
TRIGGlX MODES: One Shot: Updatcs output buffer once at first
"NlLINE COMMANDS: IFC, REN, EOI, SRQ, ATN.
valid conversion after triggeronTALKand/arG~T. Continuous:
PROGRAMMABLE PARAMETERr3 Updates output buffer at al, valid ~onwr~ion~ after trigger.
Front Panel Controls: Range, Filter, Zero, Damping, Hi
Resoluli"".
Internal Pnrametea: SRQ Rcsponsc, Trigger Modes, Data Ter-
minators.
GENERAL
DISPLAY: Seven 0.5 in. LED digits with appropriate decimal point INFUT CONNECTOR% Special low thermal for 2"OmV and lower
and polarity. ranges. Binding posts for 2V to lOO"V ranges.
NOISE: <30"" p-p on lowest range wit,, Filter on. DIMENSIONS, WEIGHT: 127mm high x 21hmm wide x 359mm
lNP"T CAPACITANCE: 5OOOpF mV ranges.
on deep (5 in. x 8.5 in. x 14.125 in.). Net weight 3.85kg (8.5 Ibs.).
SETTLING TIME: 0.5 sec. to within 25 COLMSof final reading with ACCESSORY SUPPLIED: Model ,506 Low `Thermal fnput Cable.
Fikr on, Damping off. ACCESSORfES AVAILABLE:
FILTER: 3-polcdigital; RC = 0.5.l.or2 wands dcpcndingonrangc. Model 262: Low Thermal Voltage Divider
CON"CRSlON WEBI): 4 readings/second. Model 1019A-1: 5%-i,,. Single Fixed Rack Mounting Kit
Model 1019A-2: 5'/rin. Dual Fixed Rack Mounting Kit
OVERLOAD INDICATION: Display indicates polarity and OFI.0. Model 1019SI: 5',~-in. Single Slide Rack Mounting Kit
ANALOG OUTPUT: Accuracy: +(0.15% of displayed reading + Model 1019S.2: Y/1-in. Dual Slide Rack Mounting Kit
ImW Time Constant: 400mscc.Level: f2V full scale on all rongcs; Model 1483: Low Thermal Connechan Kit
xl or xl000 gain. Model 1484: &fill Kit for 1483Kit
ISOLATION: Input LO to Output LO or power line ground: ,400" Madcl1485: Fcmalc Low Thermal Input Conneck!r
peak, 5 x IOW*Hz, >I09 paralleled by 150OpF. Model 1486: Male Low Thermal Input Connector
WARM-UP: 1 hour to r&cd accuracy. Model 1488: Low Thermal Shorting I'lug
ENVIRONMENTAL LIMITS: Operating: 0"35"C, O%-80% *da- Made, 1506: Low Thermal Input Cable (4 ft., clips)
tive humidity. Storage: -25" to +WC. Model 1507: Low Thermal Input Cable (4 ft., plugs)
Model ,815: Maintcnancc Kit
POWER: 105-123' or 21~25OV (internal switch selected), 50-6OHz, Model 8003: Low Resistance Test Fixture
30Vh maximum.
TABLE OF CONTENTS
Paragraph Title Page
SECTION l-GENERAL INFORMATION
1.1 Introduction.. ............................................................................... l-l
1.2 Model181 Features.. ......................................................................... 1~1
1.3 Optional Accessories .......................................................................... 1~1
1.4 Warranty Information ......................................................................... l-l
1.5 ManualAddenda.. ........................................................................... 1~2
1.6 SafetySymbolsandTerms ..................................................................... 1~2
1.7 ScopeofOperator'sManual...........................................................~.~~ ..... 1~2
1.8 Specifications ................................................................................ 1-2
SECTION 2-OPERATION
2.1 Introduction.. ................................................................................ 2~1
2.2 Unpackingandlnspection ............................................................ ........ 2~1
2.3 PreparingforOperation.................................................................~...~ ... 2~1
2.4 Operating Conrtrolsand Connections ............................................................. 2-2
2.5 BasicVoltageMeasurement..........................................................~.~ ........ 2-2
2.6 Nanovolt and Microvolt Measurements ....................................................... ... 2~4
2.7 Special Measuring Situations .................................................................... 2-4
2.8 Additional Front Panel Controls. ................................................................. 2-5
2.9 UsingtheAnalogOutput ....................................................................... 2~6
2.10 Source Resistance Considerations .......................................................... .... 2-J
2.11 Microvolt and Nanovolt Measurement Consideration ............................................... 2-8
SECTION 3-APPLICATIONS
3.1 Introduction.. ................................................................................ 3-1
3.2 StandardCellComparisons ..................................................................... 3~1
3.3 Low Resistance "Lindeck" Measurements ..................................................... .. 3-2
3.4 TemperatureMeasurements ......................................................... .......... 3-3
3.5 ResistanceThermometry ....................................................................... 3~3
3.6 SemiconductorTesting ......................................................................... 3-3
3.7 JosephsonJunctionStudies .................................................................... 3-3
SECTION 4-IEEE OPERATION
4.1 Introduction to the IEEE-488 Bus ................................................................. 4-1
4.2 Descriptionof BusLines ........................................................................ 4-l
4.3 IEEE-488Set-UpProcedure.. ................................................................... 4-2
4.4 BusCommands.. ............................................................................. 4-3
4.6 Device-Dependentcommands .................................................................. 4~5
4.6 Data Format.. ................................................................................ 4-7
4.7 StatusByte Format.. .......................................................................... 4-8
4.8 StatusWordFormat ........................................................................... 4-9
4.9 ProgrammingExample ......................................................................... 4-9
LIST OF ILLUSTRATIONS
Figure Title Page
l-l Model 181 Front Panel View. ............. ...... ...... ...... .......... l-2
2-1 Line Voltage Switch Location ............ ...... ...... ...... ...... .......... 2-l
2-2 Front Panel Controls and Connections ..... ...... .......... 2-3
2-3 Rear Panel Controls and Connections. ..... ...... ...... ...... .......... 2-3
2-4 Basic Voltage Measurements. ............ 2-4
2-5 mV and nV Measurements ............... ...... ...... ...... 2-4
2-6 Common Ground Connections for V and mV ...... .......... 2-5
2-7 Filter Response Graph. .................. ...... ...... ...... ...... .......... 2-6
2-8 Analog Output Connections .............. ...... ...... ...... .......... 2-6
2-9 Xl000 Analog Output ................... ...... ...... ...... ...... .......... 2-7
2-10 Source Resistance Consideration ......... ...... ...... ...... .......... 2-7
2-11 Thermal emf Generation ................. ...... ...... ...... ...... .......... 2-8
2-12 Power Line Ground Loops ............... ...... ...... 2-9
2-13 Ground Loop Voltage Generation ......... ...... ...... ...... ...... .......... 2-9
2-14 Eliminating Ground Loops ............... ...... ...... ...... ...... ......... 2-10
3-l Standard Cell Comparison ............... ...... ...... ...... ...... .......... 3-l
3-2 Absolute Cell Measurement Connections. .. ...... ...... ...... ...... 3-2
3-3 Low Resistance Measurement Connections. ...... ...... ...... ...... .......... 3-2
3-4 Minimizing Josephson Junction RFI Effects ...... ...... ...... ...... .......... 3-3
4-1 IEEE Bus Configuration .................. ...... ...... ...... ...... .......... 4-l
4-2 IEEEHandshakeSequence ............... ...... ...... ...... ...... .......... 4-2
4-3 Primary Address and IEEE Mode Switches. ...... ...... ...... ...... 4-3
4-4 IEEE Contact Configuration .............. ...... ...... ...... ...... .......... 4-3
4-5 IEEE Bus Data Format ................... ...... ...... .......... 4-8
4-6 Status Byte Format ..................... ...... ...... ...... .......... 4-9
4-7 Programming Example .................. ...... ...... ...... ......... 4-11
4-8 Timing Diagram ....................... ..... ..... ..... ..... 4-13
LIST OF TABLES
Table Page
2-l Fuse Selection ......................... ...... 2-l
2-2 Settling Times ......................... ...... 2-6
2-3 Analog Output Parameters. .............. ...... 2-7
4-l IEEE Contact Designations ............... ...... 4-4
4-2 Bus Command Summary ................ ...... 4-4
4-3 Device-Dependent Command Summary ... 4-6
4-4 Range Commands ...................... ...... 4-6
4-5 Default Conditions. ..................... ...... 4-7
4-6 Data String Exponent Values ............. ...... 4-8
4-7 Error and Data Code Summary ........... 4-9
4-8 Status Word Example ................... ...... 4-9
4-S HP-85 BASIC IEEE-488 Statements ....... ., .... 4-10
ii
SECTION 1
GENERAL INFORMATION
1.1 INTRODUCTION 1 Model 1483 Low-Thermal Connection Kit. The Model
1483 kit contains a crimp tool, pure copper lugs, Lowe
The Keithley Model 181 is a highly sensitive nanovoltmeter thermal cadmium solder, copper alligator clips, and
with a large, easy to read 5 K or 6 `/ digit display. The Model assorted hardware. It may be used for constructing ex-
181 is unique in that it combines microprocessor technology perimental circuits with low-thermal connections to
with a new concept in low-noise, high-gain front ends, minimize thermal emf effects.
resulting in a programmable instrument with sensitivity
2 Model 1484 Refill Kit. The Model 1484 kit contains
down to 10nV. The Model 181 provides highly accurate,
replacement parts for the Model 1483.
stable, low-noise readings on seven ranges for DC voltage
measurements between 1OnV and 1OOOV. The mV ranges 3 Model 1485 Low-Thermal Female Connector. The
use a special low-thermal input connector, while connec- Model 1485 connector is used for the mV INPUT on the
tions for the higher voltage ranges are made through two front panel of the Model 181.
S-way binding posts. Additional versatility is afforded by the 4 Model 1486 Low-Thermal Male Connector. The Model
inclusion of an IEEE-488 interface which allows the unit to 1486 connector mates with the Model 1485 female con-
communicate with other instrumentation. nector. It can be used to construct custom cables of
various lengths. This connector is used with the Model
1.2 MODEL 181 FEATURES 1506 and 1507.
5 Model 1488 Low-Thermal Shorting Plug. The Model
The Model 181 includes the following features: 1488 provides a means of shorting the mV INPUT to
l High Sensitivity. The resolution of the Model 181 on the check instrument offset and drift.
2mV range is IO ~* volts (10nV). 6 Model 1503 LawThermal Solder. The Model 1503 kit
l 5% or 6% Digit Resolution. Normal 5% digit display contains low-thermal cadmium solder to make solder
resolution may be increased to 6 % digits at the touch of a connections for low voltage measurements.
button. 7 Model 1506 Low-Thermal Input Cable. The Model 1506
l IEEE-488 Interface. A built in IEEE-488 interface allows cable is supplied with the unit It is a specially designed,
the instrument to communicate with other devices such four foot triaxial cable that provides excellent shielding
as a central controller or printer. for sensitive measurements. The Model 1506 has two
l Analog Output. An analog output, which accurately color coded alligator clips on one end, and a Model
reflects the displayed readings, is available from the rear 1486 low-thermal male connector at the other end.
pallel. 8 Model 1507 Low-Thermal Cable. The Model 1507 cable
l 3-p& Digital Filter. The internal 3-p& filter minimizes the is similar to the Model 1506. except that the alligator
effects of noise in voltage readings and may be controlled clips are replaced by spade lugs. The Model 1507 is cons
from the front panel or IEEE bus. strutted of a four foot triaxial cable and has a Model
1486 low-thermal male connector on one end.
l Separate Inputs. A special input connector is used for the
mV ranges to minimize thermal emf generation. 9 Model 1815 Maintenance Kit. The Model 1815 kit cons
tains a calibration cover and extender cables that are
l Isolated Low Terminals. The low signal connections for
helpful when making service adjustments to the Model
both inputs are isolated from power line ground and 181. The calibration cover replaces the top cover while
from IEEE low to minimize ground loop problems.
making these adjustments. The extender cables allow
l Color Coded Front Panel. Inputs, range switches, and individual PC cards to be partially removed from the unit
other front panel controls are marked to form color-coded during maintenance.
groups for easier operation.
10 Model 1019 Rack Mounting Kit. The Model 1019 kit
allows the Model 181 to be conveniently mounted in a
1.3 OPTIONAL ACCESSORIES
standard 19 inch rack.
A summary of the many optional Model 181 accessories is 1.4 WARRANTY INFORMATION
listed in the following paragraphs. These accessories are
designed to enhance the capabilities of the instrument and
Warranty information may be found inside the front cover of
are described in more detail in the Model 181 Service
this manual. If warranty service is required, contact the
Manual, Document Number 30816. Contact the nearest
Keithley representative in your area or the factory to detw
Keithley representative or the factory to obtain accessories.
mine the correct course of action. Keithlev maintains service
l-l
facilities in the United States, West Germany, Great Britain, The WARNING used in this manual explains dangers that
France, the Netherlands, Switzerland and Austria. lnforma- could result in personal injury or death.
tion concerning the application, operation or service of your
instrument may be directed to the applications engineer at The CAUTION used in this manual explains hazards that
any of the previously mentioned locations. Check inside could damage the instrument.
front cover of this manual for addresses.
1.7 SCOPE OF OPERATOR'S MANUAL
1.5 MANUAL ADDENDA
This manual is intended to familiarize the operator with the
Because of a policy of constant improvement, it may operating controls and features of the Model 181 nanovolto-
become necessary to make changes to the unit. Any modifi- meter. Some of the items covered in this manual include:
cations will be listed in an addendum attached to the inside basic and nanovolt measurement techniques, possible pro-
back cover of this manual. Be sure to note these changes blems that could result when making measurements, addi-
before attempting to operate the instrument. tional Model 181 uses, operation of the Model 181 on the
IEEE-488 bus, and programming examples. For technical in-
1.6 SAFETY SYMBOLS AND TERMS formation including performance verification, theory of
operation, and maintenance procedures, refer to the Model
Safety symbols used in this manual are as follows: 181 Service Manual.
The symbol A on the instrument denotes that the 1.8 SPECIFICATIONS
user should refer to the operating instructions.
For Model 181 detailed specifications, refer to the specifica-
The symbol 1/2) on the ,nstrument denotes that 1OOOV tions that precede this section.
or more may be present on the terminal(s).
Figure l-l. Model 181 Front Panel View
1-2
SECTION 2
OPERATION
2.1 INTRODUCTION transformer must be installed. Contact your
Keithley representative or the factory for
This section contains information needed for basic Model information.
181 operation. Be sure to read this entire section before
attempting to operate the unit.
To remove the top cover. remove the two screws securing
2.2 UNPACKING AND INSPECTION the cover to the rear panel. Then lift off the cover from the
back until the tabs at the front of the cover clear the front
The Model 181 was carefully inspected before shipment. panel. Then remove the cover entirely.
Upon receiving the unit, unpack all the items from the ship-
ping carton and check for any damage that might have Refer to Figure 2-1 for the location of the voltage switch.
occurred during shipment. Report any damage to the ship- Set the switch to the appropriate voltage. Also make sure
ping agent at once. Save the original packing material for the proper fuse is installed; refer to Table 2-l for the proper
possible future reshipment. Contact your nearest Keithley type.
representative or the factory if the unit fails to function
properly. Replace the top cover in the reverse order. Make sure the
tabs at the front of the cover mate with the slots in the front
The following items are included with every Model 181 panel. Finally, install the two screws that secure the top
shipment: cover to the rear panel.
1. Model 181 Nanovoltmeter Table 2-1. Fuse Selection
2. Model 181 Operator's Manual
3. Model 181 Service Manual
4. Model 1506 Low-Thermal Input Cable
5. List of computer programs.
6. Additional accessories as ordered. 3AG. SLO BLO
2.3 PREPARING FOR OPERATION 3AG. SLO BLO
Before operating the Model 181, the appropriate line voltage
must be selected and the unit must be plugged into a proper POWER
power source. This section covers each of these steps; be TRANSFORMER
sure to observe any precautions that are given. REAR PANEL /
/
1. Line Voltage Selection. The operating voltage of the VOLTAGE /-
Model 181 was set at the factory as indicated on the rear SWITCH
panel. Do not attempt to operate the unit with power line
voltages outside the indicated range. If it is necessary to
change the operating voltage, the top cover of the in-
strument must be removed to allow access to the line
voltage selection switch.
WARNING
These instructions are intended for use
only by qualified service personnel. Do not
remme the top cover unless qualified to
do so because of the possibility of electric
shock.
NOTE
The Model 181 is designed to operate with
105.125V or 210.250V as selected by the inter-
nal switch. For operation on 90.IlOV and WV=
Y
FRONT
180.22OV power sources, a special power
Figure 2-l. Line Voltage Switch Location
2-l
2. Power Line Connection. The Model 181 power cord is 4. IEEE Status Lights. The TALK, LISTEN, and REMOTE in-
supplied with a 3.prong plug that is designed to be used dicator lights show the present IEEE status of the Model
with grounded outlets. Connect the female end of this 181. For complete IEEE information, refer to Section 4.
cord to the power receptacle on the rear panel of the 5. Rear Panel Controls and Connections. The rear panel
unit. Connect the other end to an appropriate power Controls and connections are shown in Figure 2-3. An
SO",lX. analog output is available through the two 5-way binding
posts. The switches and connector shown in the lower
CAUTION left corner are for use with the IEEE-488 bus. The func-
Make sure the proper line voltage is tions and operation of these connectors and switches will
selected as described in the last section. be covered in more detail in later paragraphs.
Failure to do so may result in damage to 6. Tilt Sail. The tilt bail is useful for elevating the front panel
the instrument, possibly voiding the of the instrument to a convenient height. To extend the
warranty. tilt bail, rotate it 90' away from the bottom cover; then
push the bail upward until it locks into place. To retract
3. Power-up Procedure. Once the power connections have the bail, first pull the bail down away from the front cover
been made, the unit may be turned on by depressing the to release the locking mechanism; then rotate the bail un-
front panel power switch. The Model 181 display should til it is flush with the bottom cover.
show the line frequency and software revision level (e.g.
2.5 BASIC VOLTAGE MEASUREMENT
F60 b7) for approximately one second. After that, the
unit will revert to the normal display mode. In addition,
Normal voltage measurements are made on the 2V through
the 1OOOV range indicator light should be on. This is one
IOOOV ranges. To use one of these ranges, the source to be
of the power-on default conditions that are explained
measured must be connected to the V INPUT. The follow-
more fully in paragraph 4.5.
ing paragraphs describe the basic procedure for making
these voltage measurements.
2.4 OPERATING CONTROLS AND CONNECTIONS
Turn on the Model 181 by depressing the front panel
Front Panel Controls. The front panel controls are shown power switch. As previously described, the unit should
in Figure 2-2. In addition to the power switch previously momentarily display the line frequency and software revi-
described, the Model 181 has a number of other front sion level. Allow a one hour warm-up period to obtain
panel switches. The 2mV. 20mV. and 200mV switches rated accuracy. Four hours are required for minimum
are used to select one of the mV measurement ranges. drift.
The 2V. 2OV. 2OOV. and 1OOOV switches are used to Select the desired voltage range by depressing the
select one of the normal voltage ranges. The light above appropriate range button. Select a range that can easily
the selected range will turn on when the appropriate handle the maximum voltage to be measured.
switch is depressed. Note that these switches may be Select other front panel operating modes, such as HI
superseded by IEEE commands as outlined in Section 4. RES, ZERO, DAMPING,and FILTER, as required. Refer
to paragraph 2.8 for further information on these
In addition to the range switches, the Model 181 has controls.
several other front panel controls. These include: the HI Connect the source to be measured to the V INPUT ter-
RES switch to select 5% or 6% resolution, the ZERO minals es shown in Figure 2-4. Note that circuit ground is
switch to enable baseline suppression, and the FILTER normally connected to the LO terminal, while the HI ter-
and DAMPING switches, which alter the response of the minal should be connected to the point to be measured.
internal 3-pole filter. These features will be described in
more detail in later sections. CAUTION
Do not exceed IOOOV between the HI and
Front Panel Connections. The front panel has two input LO V INPUT terminals or the instrument
connectors. The two 5.way binding posts are used for might be damaged. Note that the LO
measurements on the 2V through 1OOOV ranges, while INPUT terminal floats and is not con-
the low-thermal mV INPUT connector is used for nected to power line ground. Therefore, it
measurements on the 2mV through 200mV ranges. is important that the potential between the
When using the mV INPUT, be sure to use the supplied LO input terminal and power line ground
low-thermal cable to minimize errors caused by thermal not exceed 14OOV. or the instrument might
emfs. be damaged.
Display. The 6% digit display is used to make Model 181 WARNING
voltage readings. The display may be switched to either Observe normal safety precautions when
5% or 6% digits at the touch of a button. A leading connecting the Model 181 to potentially
minus sign appears when negative voltages are lethal voltage sources. Failure to observe
measured, and the decimal point is automatically placed. these precautions may result in serious
Overrange is indicated by an "OFLO" message. personal injury because of electric shock.
2-2
DISPLAY IEEE STATUS LIGHTS
mV RANGES
- mV INPUT
POWER DISPLAY ZERO FILTER CONTROLS V HANGES
ON/OFF RESOLUTION ENABLE
Figure 2-2. Front Panel Controls and Connections
AC RECEPTACLE
*,\
A? .,,,,,.
I ,Nt ClArlNG
,,, :r,. ,...
ANALOG 1
,. t.;,
OUTPUT ^^ ^. ,.,. ,,I_, . ",.L
II 0 I U,SLUNNC
ANALOG OUTPUT IEEE CONNECTOR
RANGE SWITCH
Figure 2.3. Rear Panel Controls and Connections
2-3
5. Observe the display; if an "OFLO" is shown, switch to 4. Connect the low-thermal cable to the mV input. Connect
the next higher range. Use the lowest range possible to the alligator clips of the cable to the voltage source to be
make the measurement. This procedure will achieve the measured as shown in Figure 2-5.
best resolution.
6. Make the voltage reading. The display shows the reading CAUTION
directly in DC volts with a leading minus sign for negative Do not exceed 120V momentary. 35V con-
voltages. No conversion is necessary as the decimal tinous, between the mV INPUT terminals.
point is automatically placed on all ranges. or 1400V between the mV low terminal and
ground. Failure to observe these precau-
7. The Model 181 input impedance is greater than 10% on tions may result in damage to the unit.
the 2V range and equal to lOMl7 on the 20V through
IOOOV ranges. Thus, loading should not be a problem 5. Observe the display reading; if the unit is in overflow,
except with very high source resistance values. Refer to select the next higher range. If an overflow condition
paragraph 2.10 for precautions to be taken under those exists on the 200mV range, use the V INPUT and appro-
conditions. priate range as outlined in the preceding paragraph.
6. Take the voltage reading. The reading may be made
directly, in millivolts, since the decimal point is
automatically placed. A leading minus sign will be
displayed for negative voltages.
7. Because of the very low signal levels involved, unwanted
7
^
no,se, as CleSCrlDea I" paragrapn z.11, may upset the ac-
VOLTAGE SOURCE
,100"
MAX, curacy of the measurement.
-
i -I- -L
-
CIRCUIT GROUND
,WHERE APPLICABLE1
Figure 2-4. Basic Voltage Measurements
2.6 NANOVOLT AND MICROVOLT MEASUREMENTS
I
The Model 161 may be used to make very low voltage
readings down to a resolution of 10nV. These readings are Figure 2-5. mV and nV Measurements
made on one of the mV ranges by using the mV INPUT on
the front panel. 2.7 SPECIAL MEASURING SITUATIONS
The following paragraphs describe the basic procedure for Some situations may call for a wide range of voltage
making these measurements. measurements that neither the V input nor mV input can
1. Turn on the Model 181 with the front panel POWER handle alone. In those cases, it may be convenient to use a
switch. Allow the unit to warm-up for at least an hour for common ground for both the V and mV inputs. Since the
rated accuracy. To guarantee low drift, allow at least four LO terminals of the mV and V inputs are internally con-
hours. nected together, it is only necessary to connect the mV Lo
2. Select the desired mV range with the appropriate front terminal (black lead of the Model 1506 low-thermal cable) to
panel switch. Use a range appropriate for the voltage to common of the circuit under test, as shown in Figure 2-6.
be measured. Using this method, either the V HI or mV HI terminal can be
3. Select other parameters such as HI RES, DAMPING, used as the test probe, depending on the voltage to be
FILTER, and ZERO as needed. Refer to paragraph 2.8 for measured.
more details on these controls.
2-4
CAUTION The zero function is especially useful for nulling out offset
Do not exceed the maximum input limit for voltages, including internal offsets of the Model 181. To use
the Model 181, especially when the mV HI the zero in this manner, short the test leads together with
terminal is connected. or damage to the in- the instrument on the desired range and depress the ZERO
strument may occur. Never parallel the mV switch; the ZERO indicator light should turn on. This stores
and V leads to prevent accidental overload the residual voltage level as the baseline. All voltage reading
to the mV input or inadvertent loading of taken with zero enabled will then be the actual voltage level
the circuit under test. since the unwanted voltage will be subtracted from the
reading.
Note that baseline suppression for the V and mV ranges
operates separately. Switching the unit between a mV and
V range, for example, will cancel the ZERO, also causing the
front panel ZERO indicator light to turn off.
Controlling the Filter. The Model 181 has an internal 3-p&
digital filter that can be controlled by the front panel FILTER
and DAMPING controls. Normally, the filter is switched on
and off as a function of the rate of change in input signal.
Depressing the FILTER button increases the RC time conk
stant of the filter. At the same time, the front panel FILTER
light will turn on. The digital filter cannot be totally disabled
by the front panel controls. However, it may be disabled by
commands given over the IEEE bus. Operating with the filter
disabled allows the user to customize Model 161 response
by using external filtering. For further information on IEEE
commands that control the filter, consult Section 4 of this
manual.
Figure 2-6. Common Ground Connection for V and mV
2.8 ADDITIONAL FRONT PANEL CONTROLS The DAMPING button controls whether or not the filter is
continuously enabled. When the DAMPING is off, the
The Model 181 has additional front panel controls that can microprocessor automatically disables the filter when the
be used to enhance the capabilities of the unit. These input voltage changes to permit rapid display update. Once
switches which include HI RES, ZERO, FILTER, and DAMP- the reading is within 25 digits of the final value on the 2mV
ING, are shown in Figure 2-2. The following paragraphs will range, and within 6 digits on the remaining ranges. the
describe the operation of these controls in more detail. microprocessor then enables the filter to minimize noise in
the final reading. When the DAMPING is on. the digital filter
HI RES. The display resolution of the Model 181 upon is permanently enabled. The unit would normally be
power-up is 5% digits. The display resolution may be in- operated in this mode only for signals that vary slowlv, or
creased to 6 % digits by depressing the HI RES switch. Once with extremely noisy ambient signals.
the unit is in the 6% digit mode, the display may be returned
to the 5% digit mode by depressing the HI RES switch a
second time. Readings made in the 5% digit mode have the Through careful use of the FILTER and DAMPING controls,
least significant digit rounded off. HI RES switch affects the user can optimize the Model 161 to the required perfw
only the data on the display; data transmitted over the IEEE mance, keeping in mind the resulting speed/ noise com-
bus always contains 6% digit information. For further infor- promises. Figure 2-7 shows four curves resulting from
mation on IEEE operation, refer to Section 4. operating the unit with various combinations of the DAMPS
ING and FILTER controls. Curve A shows the fastest
Zero. The Zero mode serves as a means for baseline sup- response time because the filter RC time constant is at a
pression. The front panel ZERO indicator light will turn on minimum. Also, with DAMPING off, the microprocessor
when the zero mode is enabled. All readings taken with the initially disables the filter as previously described.
zero enabled will be the difference between the stored
baseline and the actual voltage level. Depressing the FILTER switch as with curve 6, has little
effect on the response time since the filter is initially off.
Curves C and D, on the other hand, show that enabling the
The baseline is obtained by connecting the instrument to
DAMPING slows the response down considerably. This can
the voltage to be zeroed. For example, if the baseline
be seen in more detail in Table 2-2, which lists the settling
voltage is IOmV, all subsequent readings will have 1OmV
times of the various control combinations.
subtracted from the actual voltage level.
2-5
Table 2-2 Settling Times
~~~~~-~~~~"~~~~~
(The readings all settle to within 0.002% of the Full Range in the specified time.)
CAUTION
The potential between the analog output
LO terminal and ground must not exceed
30V. Make sure the external device does
not exceed this voltage on its common or
ground connections. Failure to observe
this precaution may damage the Model
181. possibly voiding the warranty. IEEE
common is connected to analog output
IOW.
Select the Xl or Xl000 range by using the analog output
gain switch on the rear panel. This switch is combined
with those used to set the IEEE mode in the lower left cor-
ner of the rear panel and is clearly marked. (See Figure
2.3.) In the Xl position, the most significant +2000
counts of the display reading can be covered, while the
2.9 USING THE ANALOG OUTPUT Xl000 position will change the range to cover the least
significant f2000 counts. In this manner, the entire 6%
The analog output of the Model 181 is useful for monitoring digits of the display may be represented.
the input signal with an external device such as a chart If necessary, the analog output may be zeroed with the
recorder. The analog signal is reconstructed from digital front panel ZERO control. To do so, depress the ZERO
data (supplied by the internal microprocessor) by a 12 bit button.
D/A converter. Because of this configuration, the analog
output will accurately reflect the display until an overflow The Model 181 will display an "OFLO" message when the
condition is reached. The analog output is optically isolated capability of a specific range is exceeded. When this
from the front panel LO terminal to avoid potential ground message is displayed. the analog output value will be + 2V if
loop problems. The following paragraphs describe the basic the polarity of the input voltage is positive, and -2V if the
procedure for using the analog output. input voltage polarity is negative.
1. Connect the measuring device to the two analog output
terminals on the rear panel as shown in Figure 2-8. An analog output range overflow can occur when the Model
181 analog range switch is in the Xl000 position. An exam-
ple of the analog ouput voltage under these conditions is
shown in Figure 2-9." The conditions shown are for the 2mV
range. but the output will react similarly on the other voltage
ranges if the proper scaling factor is applied. For each ten-
fold increase in voltage range, the scale of the horizontal
axis must also be multiplied by a factor of ten.
The horizontal axis of Figure 2-9 has an input voltage range
between -10&V and +lOpV. The vertical axis shows an
analog output voltage between -2V and +ZV. which is the
maximum range of the analog output. Beginning at the OV
point on the graph, the analog output follows the input
Figure 2-8. Analog Output Connections voltage linearly until the input voltage reaches +2pV. The
analog output will then suddenly switch to the maximum
negative output value of -2V. Thus, for each 4uV increment
* Units with B-7 software.
2-6
in input voltage, the output pattern repeats. ihe action of 2.10 SOURCE RESISTANCE CONSIDERATION
the analog output for negative input voltages is the same,
except that the slope of the graph is negative for these The Model 181 has an input resistance greater that IGIl
negative-going input voltages. flO% on the 2mV. 20mV. 200mV. and 2V ranges. The in-
strument will meet this input resistance specification on the
mV ranges even when in overflow with voltages up to 1V.
The input resistance on the remaining voltages ranges is
lOML2. Thus, the Model 181 input resistance is sufficiently
high to minimize loading errors in most measuring situa-
tions. For voltage sources with very high source resistance,
two precautions should be observed when using the Model
181.
Shielding becomes more critical when the source resistance
is very high. Otherwise, interference signals may be picked
up by the test leads. Noise picked up in this manner can af-
Figure 2-9. Xl000 Analog Output fect the mV ranges more severely. but shielding might be
necessary for connections to the V INPUT in extreme
By counting the number of repeating waveforms on a chart situations.
recorder, the user can easily determine the actual voltage at
the input, even though the range of the analog output was Loading of the voltage source by the Model 181 can become
exceeded. If, for example, the +lV point on the second important with high source resistance values. As the source
peak with a positive-going slope is noted, it can be clearly resistance increases, the error due to loading increases.
determined that the input voltage was +5@V at that par- Figure Z-10 shows the method used to determine the Peru
ticular time. cent error due to loading. The voltage source has an internal
resistance R,, while the internal resistance of the Model 181
A summary of analog output information is shown in Table is represented by R,. The source voltage is E, while the
2.3. Each range of input values corresponds to the incre- voltage actually measured by the meter is E,.
ment necessary to cause the output to go through its entire
0 to 2V range. Note that the sensitivity is increased by a fac- The voltage actually seen by the meter is attenuated by the
tor of a thousand on the Xl000 range. For example, when voltage-divider action of R and R, and can be found by
the Model 181 is in the 200mV range, and the analog switch using the relationship: E, = &R,IIRL + I?,).
is in the Xl position, the output voltage will swing from 0 to
2V in a smooth manner as the input voltage increases We can modify this relationship to obtain a formula for per-
gradually from 0 to 200mV. When the analog output switch cent errors as follows: Percent Error = lOOR,/(R, * R,i~
is changed to the Xl000 position, the input need only swing
between 0 and 2OOpV to obtain the same voltage swing at
the analog output. Beyond those input limitations, the out- From the above, it is obvious that the input resistance of the
put voltage will repeat as shown in Figure 2-9. Model 181 must be at least 99 times greater that the source
The output resistance of the analog ouput is Ikll for all resistance if the loading error is to be kept to 1%. This max-
voltage ranges regardless of the position of the analog range imum 1% error limitation will be achieved on the 2mV
switch. Thus, loading problems caused by external devices through 2V ranges with sources resistances up to lO.lMI1,
are minimized. To keep loading errors below I%, the input while the source resistance should be no greater than IOlklI
resistance of any device connected to the analog output if the same 1% error limitation is to be maintained on the
should be greater than lOOk0. 2OV through IOOOV ranges. If lower errors are required, the
source resistance must be correspondingly less.
Table 2-3. Analog Output Parameters Rs
INPUT FOR
IG OUTPUT
20mV
200mV
2v
20 v
200 v
1 kV
Figure 2-10. Source Resistance Considerations
*IV Full Range Maximum
2-7
2.11 MICROVOLT AND NANOVOLT MEASUREMENT Thermoelectric potentials. Thermal emf's are small electric
CONSIDERATIONS potentials generated by differences in the temperature at
the junction of two dissimi