Keithley 181 User Manual

Operator’s Manual

Model 181

Digital Nanovoltmeter

01982. Keithley Instruments, Inc Cleveland, Ohio, U.S.A.

Document Number 32421

181

NANOVOLTMETER

IEEE-488 BUS IMPLEMENTATION MULTlLlNE COMMANDS: DCL, LLCJ, xc, GET. “NlLINE COMMANDS: IFC, REN, EOI, SRQ, ATN. PROGRAMMABLE PARAMETERr3

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

and polarity.

NOISE: <30”” p-p on lowest range wit,, Filter on.

lNP”T CAPACITANCE: 5OOOpF on mV ranges.

SETTLING TIME: 0.5 sec. to within 25 COLMS of final reading with

Fikr on, Damping off. FILTER: 3-polcdigital; RC = 0.5.l.or2 wands dcpcndingonrangc. CON”CRSlON WEBI): 4 readings/second. OVERLOAD INDICATION: Display indicates polarity and OFI.0. ANALOG OUTPUT: Accuracy: +(0.15% of displayed reading +

ImW Time Constant: 400mscc. Level: f2V full scale on all rongcs;

xl or xl000 gain.

ISOLATION: Input LO to Output LO or power line ground: ,400”

peak, 5 x IOW*Hz, >I09 paralleled by 150OpF. WARM-UP: 1 hour to r&cd accuracy. ENVIRONMENTAL LIMITS: Operating: 0”35”C, O%-80% *da-

tive humidity. Storage: -25” to +WC. POWER: 105-123’ or 21~25OV (internal switch selected), 50-6OHz,

30Vh maximum.

ADDRESS MODES: TALK ONLY and ADDRESSABLE. TRIGGlX MODES: One Shot: Updatcs output buffer once at first

valid conversion after triggeronTALKand/arG~T. Continuous: Updates output buffer at al, valid ~onwr~ion~ after trigger.

INFUT CONNECTOR% Special low thermal for 2”OmV and lower

ranges. Binding posts for 2V to lOO”V ranges.

DIMENSIONS, WEIGHT: 127mm high x 21hmm wide x 359mm

deep (5 in. x 8.5 in. x 14.125 in.). Net weight 3.85kg (8.5 Ibs.). ACCESSORY SUPPLIED: Model ,506 Low ‘Thermal fnput Cable. ACCESSORfES AVAILABLE:

Model 262: Low Thermal Voltage Divider

Model 1019A-1: 5%-i,,. Single Fixed Rack Mounting Kit

Model 1019A-2: 5’/rin. Dual Fixed Rack Mounting Kit

Model 1019SI: 5’,~-in. Single Slide Rack Mounting Kit

Model 1019S.2: Y/1-in. Dual Slide Rack Mounting Kit

Model 1483: Low Thermal Connechan Kit

Model 1484:

Madcl1485: Fcmalc Low Thermal Input Conneck!r

Model 1486: Male Low Thermal Input Connector

Model 1488: Low Thermal Shorting I’lug

Made, 1506: Low Thermal Input Cable (4 ft., clips)

Model 1507:

Model ,815: Maintcnancc Kit

Model 8003: Low Resistance Test Fixture

&fill Kit for 1483 Kit

Low Thermal Input Cable (4 ft., plugs)

TABLE OF CONTENTS

Paragraph

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

2.10

2.11

3.1

3.2

3.3

3.4

3.5

3.6

3.7

Title

SECTION l-GENERAL INFORMATION

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

Model181 Features.. .........................................................................

Optional Accessories

Warranty Information .........................................................................

ManualAddenda..

SafetySymbolsandTerms .....................................................................

ScopeofOperator’sManual...........................................................~.~~ .....

Specifications ................................................................................

SECTION 2-OPERATION

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

Unpackingandlnspection

PreparingforOperation.................................................................~...~

Operating Conrtrolsand Connections

BasicVoltageMeasurement..........................................................~.~

Nanovolt and Microvolt Measurements

Special Measuring Situations

Additional Front Panel Controls. .................................................................

UsingtheAnalogOutput .......................................................................

Source Resistance Considerations ..........................................................

Microvolt and Nanovolt Measurement Consideration SECTION 3-APPLICATIONS

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

StandardCellComparisons .....................................................................

Low Resistance “Lindeck” Measurements

TemperatureMeasurements .........................................................

ResistanceThermometry .......................................................................

SemiconductorTesting .........................................................................

JosephsonJunctionStudies ....................................................................

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

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

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

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

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

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

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

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

........

..........

....

...

Page

l-l 1~1 1~1 l-l 1~2 1~2 1~2 1-2

2~1 2~1 2~1 2-2 2-2 2~4 2-4 2-5 2~6

2-J

2-8

3-1

3~1 3-2 3-3 3~3 3-3 3-3

4.1

4.2

4.3

4.4

4.6

4.7

4.8

4.9

SECTION 4-IEEE OPERATION

Introduction to the IEEE-488 Bus .................................................................

Descriptionof BusLines ........................................................................

IEEE-488Set-UpProcedure.. ...................................................................

BusCommands.. .............................................................................

Device-Dependentcommands ..................................................................

Data Format.. ................................................................................

StatusByte Format.. ..........................................................................

StatusWordFormat ...........................................................................

ProgrammingExample .........................................................................

4-1 4-l 4-2 4-3 4~5 4-7 4-8 4-9 4-9

LIST OF ILLUSTRATIONS

Figure

l-l 2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10 2-11 2-12 2-13 2-14 3-l 3-2 3-3 3-4 4-1 4-2 4-3

4-4

4-5

4-6

4-7

4-8

Model 181 Front Panel View. .............

Line Voltage Switch Location ............

Front Panel Controls and Connections Rear Panel Controls and Connections. Basic Voltage Measurements.

mV and nV Measurements ...............

Common Ground Connections for V and mV

Filter Response Graph. ..................

Analog Output Connections ..............

Xl000 Analog Output ...................

Source Resistance Consideration .........

Thermal emf Generation .................

Power Line Ground Loops ...............

Ground Loop Voltage Generation .........

Eliminating Ground Loops ...............

Standard Cell Comparison ...............

Absolute Cell Measurement Connections. Low Resistance Measurement Connections. Minimizing Josephson Junction RFI Effects

IEEE Bus Configuration ..................

IEEEHandshakeSequence ...............

Primary Address and IEEE Mode Switches.

IEEE Contact Configuration ..............

IEEE Bus Data Format ...................

Status Byte Format .....................

Programming Example ..................

Timing Diagram .......................

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

.....

Title Page

......

.....

......

.....

......

.....

......

.....

..........

.......... 2-l

.......... 2-3

.......... 2-5

.......... 2-6

.......... 2-7

.......... 2-8

.......... 2-9

......... 2-10

..........

.......... 3-2

..........

.......... 4-l

.......... 4-2

..........

.......... 4-8

.......... 4-9

......... 4-11

l-2

2-4 2-4

2-9

3-l 3-2

3-3

4-3 4-3

4-13

Table

2-l

2-2

2-3

4-l

4-2

4-3

4-4

4-5

4-6

4-7

4-8

4-S

Fuse Selection .........................

Settling Times .........................

Analog Output Parameters. ..............

IEEE Contact Designations ...............

Bus Command Summary ................

Device-Dependent Command Summary

Range Commands ......................

Default Conditions. .....................

Data String Exponent Values .............

Error and Data Code Summary ...........

Status Word Example ...................

HP-85 BASIC IEEE-488 Statements .......

LIST OF TABLES

...

......

., ....

Page

2-l 2-6 2-7 4-4 4-4 4-6 4-6 4-7 4-8 4-9 4-9

4-10

SECTION 1

GENERAL INFORMATION

1.1 INTRODUCTION The Keithley Model 181 is a highly sensitive nanovoltmeter

with a large, easy to read 5 K or 6 ‘/ digit display. The Model 181 is unique in that it combines microprocessor technology with a new concept in low-noise, high-gain front ends, resulting in a programmable instrument with sensitivity down to 10nV. The Model 181 provides highly accurate, stable, low-noise readings on seven ranges for DC voltage measurements between 1OnV and 1OOOV. The mV ranges use a special low-thermal input connector, while connections for the higher voltage ranges are made through two S-way binding posts. Additional versatility is afforded by the inclusion of an IEEE-488 interface which allows the unit to communicate with other instrumentation.

1.2 MODEL 181 FEATURES The Model 181 includes the following features:

High Sensitivity. The resolution of the Model 181 on the

2mV range is IO ~* volts (10nV).

5% or 6% Digit Resolution. Normal 5% digit display

resolution may be increased to 6 % digits at the touch of a button.

IEEE-488 Interface. A built in IEEE-488 interface allows the instrument to communicate with other devices such as a central controller or printer.

Analog Output. An analog output, which accurately

reflects the displayed readings, is available from the rear

pallel.

3-p& Digital Filter. The internal 3-p& filter minimizes the

effects of noise in voltage readings and may be controlled from the front panel or IEEE bus.

Separate Inputs. A special input connector is used for the

mV ranges to minimize thermal emf generation.

Isolated Low Terminals. The low signal connections for

both inputs are isolated from power line ground and

from IEEE low to minimize ground loop problems.

Color Coded Front Panel. Inputs, range switches, and

other front panel controls are marked to form color-coded

groups for easier operation.

1.3 OPTIONAL ACCESSORIES

Model 1483 Low-Thermal Connection Kit. The Model

1483 kit contains a crimp tool, pure copper lugs, Lowe thermal cadmium solder, copper alligator clips, and assorted hardware. It may be used for constructing experimental circuits with low-thermal connections to minimize thermal emf effects.

Model 1484 Refill Kit. The Model 1484 kit contains replacement parts for the Model 1483.

Model 1485 Low-Thermal Female Connector. The Model 1485 connector is used for the mV INPUT on the front panel of the Model 181.

Model 1486 Low-Thermal Male Connector. The Model 1486 connector mates with the Model 1485 female connector. It can be used to construct custom cables of various lengths. This connector is used with the Model 1506 and 1507.

Model 1488 Low-Thermal Shorting Plug. The Model 1488 provides a means of shorting the mV INPUT to check instrument offset and drift.

Model 1503 LawThermal Solder. The Model 1503 kit

contains low-thermal cadmium solder to make solder connections for low voltage measurements.

Model 1506 Low-Thermal Input Cable. The Model 1506 cable is supplied with the unit It is a specially designed, four foot triaxial cable that provides excellent shielding for sensitive measurements. The Model 1506 has two color coded alligator clips on one end, and a Model 1486 low-thermal male connector at the other end.

Model 1507 Low-Thermal Cable. The Model 1507 cable is similar to the Model 1506. except that the alligator clips are replaced by spade lugs. The Model 1507 is cons strutted of a four foot triaxial cable and has a Model 1486 low-thermal male connector on one end.

Model 1815 Maintenance Kit. The Model 1815 kit cons tains a calibration cover and extender cables that are helpful when making service adjustments to the Model

181. The calibration cover replaces the top cover while making these adjustments. The extender cables allow individual PC cards to be partially removed from the unit during maintenance.

Model 1019 Rack Mounting Kit. The Model 1019 kit allows the Model 181 to be conveniently mounted in a standard 19 inch rack.

A summary of the many optional Model 181 accessories is listed in the following paragraphs. These accessories are designed to enhance the capabilities of the instrument and are described in more detail in the Model 181 Service Manual, Document Number 30816. Contact the nearest Keithley representative or the factory to obtain accessories.

1.4 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 detw

mine the correct course of action. Keithlev maintains service

l-l

facilities in the United States, West Germany, Great Britain,

France, the Netherlands, Switzerland and Austria. lnforma-

tion concerning the application, operation or service of your

instrument may be directed to the applications engineer at any of the previously mentioned locations. Check inside front cover of this manual for addresses.

1.5 MANUAL ADDENDA

Because of a policy of constant improvement, it may

become necessary to make changes to the unit. Any modifi-

cations will be listed in an addendum attached to the inside

back cover of this manual. Be sure to note these changes

before attempting to operate the instrument.

1.6 SAFETY SYMBOLS AND TERMS

Safety symbols used in this manual are as follows:

The WARNING used in this manual explains dangers that could result in personal injury or death.

The CAUTION used in this manual explains hazards that could damage the instrument.

1.7 SCOPE OF OPERATOR’S MANUAL

This manual is intended to familiarize the operator with the operating controls and features of the Model 181 nanovolto-

meter. Some of the items covered in this manual include: basic and nanovolt measurement techniques, possible problems that could result when making measurements, addi-

tional Model 181 uses, operation of the Model 181 on the

IEEE-488 bus, and programming examples. For technical information including performance verification, theory of operation, and maintenance procedures, refer to the Model

181 Service Manual.

The symbol

A on the instrument denotes that the

user should refer to the operating instructions.

The symbol 1/2)

on the ,nstrument denotes that 1OOOV

or more may be present on the terminal(s).

1.8 SPECIFICATIONS

For Model 181 detailed specifications, refer to the specifications that precede this section.

1-2

Figure l-l. Model 181 Front Panel View

SECTION 2

OPERATION

2.1 INTRODUCTION This section contains information needed for basic Model

181 operation. Be sure to read this entire section before

attempting to operate the unit.

2.2 UNPACKING AND INSPECTION The Model 181 was carefully inspected before shipment.

Upon receiving the unit, unpack all the items from the shipping carton and check for any damage that might have occurred during shipment. Report any damage to the shipping agent at once. Save the original packing material for

possible future reshipment. Contact your nearest Keithley

representative or the factory if the unit fails to function

properly. The following items are included with every Model 181

shipment:

1. Model 181 Nanovoltmeter

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.

2.3 PREPARING FOR OPERATION

transformer must be installed. Contact your Keithley representative or the factory for information.

To remove the top cover. remove the two screws securing 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 panel. Then remove the cover entirely.

Refer to Figure 2-1 for the location of the voltage switch. Set the switch to the appropriate voltage. Also make sure

the proper fuse is installed; refer to Table 2-l for the proper

type. 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 panel. Finally, install the two screws that secure the top cover to the rear panel.

Table 2-1. Fuse Selection

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 source. This section covers each of these steps; be sure to observe any precautions that are given.

1. Line Voltage Selection. The operating voltage of the Model 181 was set at the factory as indicated on the rear 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 instrument 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

180.22OV power sources, a special power

POWER

TRANSFORMER

REAR PANEL

FRONT

WV=

VOLTAGE VOLTAGE

SWITCH SWITCH

Figure 2-l. Line Voltage Switch Location

2-l

Power Line Connection. The Model 181 power cord is supplied with a 3.prong plug that is designed to be used with grounded outlets. Connect the female end of this cord to the power receptacle on the rear panel of the unit. Connect the other end to an appropriate power SO”,lX.

CAUTION

Make sure the proper line voltage is selected as described in the last section.

Failure to do so may result in damage to the instrument,

possibly voiding the

warranty.

Power-up Procedure. Once the power connections have been made, the unit may be turned on by depressing the front panel power switch. The Model 181 display should show the line frequency and software revision level (e.g.

F60 b7) for approximately one second. After that, the

unit will revert to the normal display mode. In addition, the 1OOOV range indicator light should be on. This is one of the power-on default conditions that are explained

more fully in paragraph 4.5.

2.4 OPERATING CONTROLS AND CONNECTIONS Front Panel Controls. The front panel controls are shown

in Figure 2-2. In addition to the power switch previously described, the Model 181 has a number of other front panel switches. The 2mV. 20mV. and 200mV switches are used to select one of the mV measurement ranges. The 2V. 2OV. 2OOV. and 1OOOV switches are used to select one of the normal voltage ranges. The light above the selected range will turn on when the appropriate switch is depressed. Note that these switches may be superseded by IEEE commands as outlined in Section 4.

In addition to the range switches, the Model 181 has several other front panel controls. These include: the HI RES switch to select 5% or 6% resolution, the ZERO

switch to enable baseline suppression, and the FILTER and DAMPING switches, which alter the response of the internal 3-pole filter. These features will be described in more detail in later sections.

Front Panel Connections. The front panel has two input connectors. The two 5.way binding posts are used for measurements on the 2V through 1OOOV ranges, while the low-thermal mV INPUT connector is used for measurements on the 2mV through 200mV ranges.

When using the mV INPUT, be sure to use the supplied

low-thermal cable to minimize errors caused by thermal emfs.

Display. The 6% digit display is used to make Model 181 voltage readings. The display may be switched to either

5% or 6% digits at the touch of a button. A leading

minus sign appears when negative voltages are measured, and the decimal point is automatically placed. Overrange is indicated by an “OFLO” message.

4. IEEE Status Lights. The TALK, LISTEN, and REMOTE indicator lights show the present IEEE status of the Model

181. For complete IEEE information, refer to Section 4.

5. Rear Panel Controls and Connections. The rear panel Controls and connections are shown in Figure 2-3. An analog output is available through the two 5-way binding posts. The switches and connector shown in the lower left corner are for use with the IEEE-488 bus. The functions and operation of these connectors and switches will be covered in more detail in later paragraphs.

6. Tilt Sail. The tilt bail is useful for elevating the front panel of the instrument to a convenient height. To extend the

tilt bail, rotate it 90’ away from the bottom cover; then

push the bail upward until it locks into place. To retract the bail, first pull the bail down away from the front cover

to release the locking mechanism; then rotate the bail until it is flush with the bottom cover.

2.5 BASIC VOLTAGE MEASUREMENT Normal voltage measurements are made on the 2V through

IOOOV ranges. To use one of these ranges, the source to be

measured must be connected to the V INPUT. The follow-

ing paragraphs describe the basic procedure for making

these voltage measurements.

Turn on the Model 181 by depressing the front panel power switch. As previously described, the unit should momentarily display the line frequency and software revision level. Allow a one hour warm-up period to obtain rated accuracy. Four hours are required for minimum drift.

Select the desired voltage range by depressing the appropriate range button. Select a range that can easily handle the maximum voltage to be measured.

Select other front panel operating modes, such as HI RES, ZERO, DAMPING,and FILTER, as required. Refer to paragraph 2.8 for further information on these controls.

Connect the source to be measured to the V INPUT terminals es shown in Figure 2-4. Note that circuit ground is normally connected to the LO terminal, while the HI terminal should be connected to the point to be measured.

CAUTION

Do not exceed IOOOV between the HI and LO V INPUT terminals or the instrument might be damaged. Note that the LO INPUT terminal floats and is not connected to power line ground. Therefore, it is important that the potential between the LO input terminal and power line ground not exceed 14OOV. or the instrument might be damaged.

WARNING

Observe normal safety precautions when connecting the Model 181 to potentially

lethal voltage sources. Failure to observe these precautions may result in serious personal injury because of electric shock.

2-2

mV RANGES

DISPLAY

IEEE STATUS LIGHTS

- mV INPUT

ANALOG

OUTPUT

POWER

ON/OFF

II I

RESOLUTION

Figure 2-2. Front Panel Controls and Connections

^ ^ ^ .

U,SLUNNC

DISPLAY

* ,\

ZERO

FILTER CONTROLS

V HANGES

ENABLE

AC RECEPTACLE

I ,Nt ClArlNG

.,,,,,. ,,, :r,. ,...

,. t.;,

,.,. ,,I_, . “,.L

ANALOG OUTPUT

RANGE SWITCH

IEEE CONNECTOR

Figure 2.3. Rear Panel Controls and Connections

2-3

5. Observe the display; if an “OFLO” is shown, switch to the next higher range. Use the lowest range possible to make the measurement. This procedure will achieve the best resolution.

6. Make the voltage reading. The display shows the reading directly in DC volts with a leading minus sign for negative voltages. No conversion is necessary as the decimal point is automatically placed on all ranges.

7. The Model 181 input impedance is greater than 10% on

the 2V range and equal to lOMl7 on the 20V through

IOOOV ranges. Thus, loading should not be a problem except with very high source resistance values. Refer to paragraph 2.10 for precautions to be taken under those conditions.

VOLTAGE SOURCE

,100” MAX,

CIRCUIT GROUND

,WHERE APPLICABLE1

-L

-I-

4. Connect the low-thermal cable to the mV input. Connect the alligator clips of the cable to the voltage source to be measured as shown in Figure 2-5.

CAUTION

Do not exceed 120V momentary. 35V con-

tinous, between the mV INPUT terminals.

or 1400V between the mV low terminal and ground. Failure to observe these precau-

tions may result in damage to the unit.

5. Observe the display reading; if the unit is in overflow, select the next higher range. If an overflow condition exists on the 200mV range, use the V INPUT and appropriate range as outlined in the preceding paragraph.

6. Take the voltage reading. The reading may be made directly, in millivolts,

automatically placed. A leading minus sign will be displayed for negative voltages.

7. Because of the very low signal levels involved, unwanted no,se, as CleSCrlDea I” paragrapn z.11, may upset the ac-

curacy of the measurement.

since the decimal point is

Figure 2-4. Basic Voltage Measurements

NANOVOLT AND MICROVOLT MEASUREMENTS

2.6

The Model 161 may be used to make very low voltage

readings down to a resolution of 10nV. These readings are made on one of the mV ranges by using the mV INPUT on

the front panel.

The following paragraphs describe the basic procedure for making these measurements.

1. Turn on the Model 181 with the front panel POWER switch. Allow the unit to warm-up for at least an hour for rated accuracy. To guarantee low drift, allow at least four hours.

2. Select the desired mV range with the appropriate front panel switch. Use a range appropriate for the voltage to be measured.

3. Select other parameters such as HI RES, DAMPING, FILTER, and ZERO as needed. Refer to paragraph 2.8 for more details on these controls.

2-4

Figure 2-5. mV and nV Measurements

2.7 SPECIAL MEASURING SITUATIONS

Some situations may call for a wide range of voltage measurements that neither the V input nor mV input can handle alone. In those cases, it may be convenient to use a

common ground for both the V and mV inputs. Since the

LO terminals of the mV and V inputs are internally connected together, it is only necessary to connect the mV Lo

terminal (black lead of the Model 1506 low-thermal cable) to

common of the circuit under test, as shown in Figure 2-6. Using this method, either the V HI or mV HI terminal can be used as the test probe, depending on the voltage to be measured.

CAUTION

Do not exceed the maximum input limit for the Model 181, especially when the mV HI terminal is connected. or damage to the in-

strument may occur. Never parallel the mV

and V leads to prevent accidental overload

to the mV input or inadvertent loading of

the circuit under test.

Figure 2-6. Common Ground Connection for V and mV

The zero function is especially useful for nulling out offset voltages, including internal offsets of the Model 181. To use the zero in this manner, short the test leads together with the instrument on the desired range and depress the ZERO switch; the ZERO indicator light should turn on. This stores the residual voltage level as the baseline. All voltage reading taken with zero enabled will then be the actual voltage level 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.

2.8 ADDITIONAL FRONT PANEL CONTROLS The Model 181 has additional front panel controls that can

be used to enhance the capabilities of the unit. These

switches which include HI RES, ZERO, FILTER, and DAMP-

ING, are shown in Figure 2-2. The following paragraphs will

describe the operation of these controls in more detail.

HI RES. The display resolution of the Model 181 upon power-up is 5% digits. The display resolution may be increased to 6 % digits by depressing the HI RES switch. Once 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

least significant digit rounded off. HI RES switch affects

only the data on the display; data transmitted over the IEEE

bus always contains 6% digit information. For further infor-

mation on IEEE operation, refer to Section 4.

Zero. The Zero mode serves as a means for baseline sup-

pression. The front panel ZERO indicator light will turn on

when the zero mode is enabled. All readings taken with the

zero enabled will be the difference between the stored

baseline and the actual voltage level.

The baseline is obtained by connecting the instrument to the voltage to be zeroed. For example, if the baseline

voltage is IOmV, all subsequent readings will have 1OmV

subtracted from the actual voltage level.

The DAMPING button controls whether or not the filter is continuously enabled. When the DAMPING is off, the microprocessor automatically disables the filter when the input voltage changes to permit rapid display update. Once the reading is within 25 digits of the final value on the 2mV range, and within 6 digits on the remaining ranges. the microprocessor then enables the filter to minimize noise in the final reading. When the DAMPING is on. the digital filter is permanently enabled. The unit would normally be operated in this mode only for signals that vary slowlv, or with extremely noisy ambient signals.

Through careful use of the FILTER and DAMPING controls, the user can optimize the Model 161 to the required perfw mance, keeping in mind the resulting speed/ noise compromises. Figure 2-7 shows four curves resulting from operating the unit with various combinations of the DAMPS

ING and FILTER controls. Curve A shows the fastest response time because the filter RC time constant is at a minimum. Also, with DAMPING off, the microprocessor initially disables the filter as previously described.

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

DAMPING slows the response down considerably. This can be seen in more detail in Table 2-2, which lists the settling times of the various control combinations.

2-5

Table 2-2 Settling Times

~~~~~-~~~~“~~~~~

(The readings all settle to within 0.002% of the Full Range in the specified time.)

2.9 USING THE ANALOG OUTPUT

The analog output of the Model 181 is useful for monitoring

the input signal with an external device such as a chart recorder. The analog signal is reconstructed from digital data (supplied by the internal microprocessor) by a 12 bit

D/A converter. Because of this configuration, the analog output will accurately reflect the display until an overflow condition is reached. The analog output is optically isolated from the front panel LO terminal to avoid potential ground loop problems. The following paragraphs describe the basic procedure for using the analog output.

1. Connect the measuring device to the two analog output terminals on the rear panel as shown in Figure 2-8.

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 Xl000 position will change the range to cover the least significant f2000 counts. In this manner, the entire 6% digits of the display may be represented.

If necessary, the analog output may be zeroed with the front panel ZERO control. To do so, depress the ZERO button.

The Model 181 will display an “OFLO” message when the

capability of a specific range is exceeded. When this message is displayed. the analog output value will be + 2V if the polarity of the input voltage is positive, and -2V if the input voltage polarity is negative.

An analog output range overflow can occur when the Model

181 analog range switch is in the Xl000 position. An example 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.

2-6

Figure 2-8. Analog Output Connections

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 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.

in input voltage, the output pattern repeats. ihe action of the analog output for negative input voltages is the same, except that the slope of the graph is negative for these negative-going input voltages.

Figure 2-9. Xl000 Analog Output

By counting the number of repeating waveforms on a chart

recorder, the user can easily determine the actual voltage at the input, even though the range of the analog output was exceeded. If, for example, the +lV point on the second

peak with a positive-going slope is noted, it can be clearly determined that the input voltage was +5@V at that particular time.

A summary of analog output information is shown in Table

2.3. Each range of input values corresponds to the incre-

ment necessary to cause the output to go through its entire 0 to 2V range. Note that the sensitivity is increased by a fac-

tor of a thousand on the Xl000 range. For example, when

the Model 181 is in the 200mV range, and the analog switch

is in the Xl position, the output voltage will swing from 0 to 2V in a smooth manner as the input voltage increases

gradually from 0 to 200mV. When the analog output switch

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-

put voltage will repeat as shown in Figure 2-9.

The output resistance of the analog ouput is Ikll for all

voltage ranges regardless of the position of the analog range

switch. Thus, loading problems caused by external devices

are minimized. To keep loading errors below I%, the input

resistance of any device connected to the analog output

should be greater than lOOk0.

Table 2-3. Analog Output Parameters

2.10 SOURCE RESISTANCE CONSIDERATION The Model 181 has an input resistance greater that IGIl

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 situations. 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 affect the mV ranges more severely. but shielding might be necessary for connections to the V INPUT in extreme situations.

Loading of the voltage source by the Model 181 can become

important with high source resistance values. As the source resistance increases, the error due to loading increases.

Figure Z-10 shows the method used to determine the Peru

cent error due to loading. The voltage source has an internal resistance R,, while the internal resistance of the Model 181 is represented by R,. The source voltage is E, while the voltage actually measured by the meter is E,.

The voltage actually seen by the meter is attenuated by the voltage-divider action of R and R, and can be found by using the relationship: E, = &R,IIRL + I?,).

We can modify this relationship to obtain a formula for per-

cent errors as follows: Percent Error = lOOR,/(R, * R,i~

From the above, it is obvious that the input resistance of the Model 181 must be at least 99 times greater that the source resistance if the loading error is to be kept to 1%. This maximum 1% error limitation will be achieved on the 2mV through 2V ranges with sources resistances up to lO.lMI1, while the source resistance should be no greater than IOlklI if the same 1% error limitation is to be maintained on the 2OV through IOOOV ranges. If lower errors are required, the source resistance must be correspondingly less.

20mV

200mV

20 v

200 v

1 kV

*IV Full Range Maximum

INPUT FOR

IG OUTPUT

Figure 2-10. Source Resistance Considerations

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