Tektronix DM 502 Instruction Manual

Tektronix, Inc.

P.0. Box 500

Beaverton, Oregon 97005

TEKTRONIX®

DM

502

DIGITAL

MULTIMETER

INSTRUCTION

MANUAL

Serial Number

070 1726

00

Flr»l Printing

Aug 1974

WARRANTY

All TEKTRONIX instruments are warranted against

defective materials and workmanship for one year.

Any questions with respect

to

the warranty should

be taken up with yourTEKTRONIX Field Engineer or

representative.

All requests for repairs and replacement parts

should

be

directed to the TEKTRONIX Field Office

or representative in your area. This will assure

you

the fastest possible

service.

Please include

the

instrument Type Number or Part Number and Serial

Number with all requests for parts or service.

Specifications and price change privileges reserved.

Copyright

c

1974 by Tektronix. Inc., Beaverton,

Oregon. Printed in the United States of America. All

rights

reserved.

Contents of this publication may not

be

reproduced in any form without permission of

Tektronix. Inc.

U.S.A. and foreign TEKTRONIX products covered

by

U.S.

and foreign patents and/or patents pending.

TEKTRONIX is a registered

trademark

of Tektronix,

Inc.

DM 502

SECTION

1

SECTION

2

SECTION

3

SECTION

4

SECTION 5

CHANGE INFORMATION

TABLE

OF

CONTENTS

Page

OPERATING INSTRUCTIONS

Introduction 1-1

Instrument Description 1-1

Installation and

Removal

1-1

Operating Considerations 1-2

Input

Connections

1-2

Functions Available

at

Rear

Connector

1-2

Sine Wave Response 1-3

Temperature Measurements 1-3

Making

Measurements

1-4

DC Voltage

Measurements

1-4

DC Current

Measurements

1-4

AC Voltage,

Current and Resistance Measurements

1-5

Decibel Measurements

1-5

Temperature Measurements

1-5

Using

a

Transistor as a

Temperature-Sensing Device

1-5

Accurate Current

Source

1-6

Specifications

1-6

THEORY OF

OPERATION

Introduction

2-1

Attenuator and Input Switching

2-1

AC Converter

2-1

DB

Converter

2-1

Ohms

Converter

2-2

Temperature Measurement Circuitry

2-2

Clock

2-3

Analog-To-Digital

Converter

2-3

Display

2-5

Power Supplies

2-5

ELECTRICAL REPLACEABLE

PARTS LIST

SERVICE INFORMATION

Internal

Adjustment Procedure

Rear Interface

Connector Assignments

Rear

Interface Connector Wire

Locations

Front Panel Controls

and Connectors

Block Diagram

Parts Location Grid

Attenuators

and Conversion Circuits

Schematic

Integrator. A/D Converter and Display

Schematic

Power Supply and Switch

Logic Schematic

MECHANICAL

REPLACEABLE PARTS LIST

Accessories

Repackaging

Exploded View

REV.

A. OCT. 1974

i

Section 1—DM 502

OPERATING

INSTRUCTIONS

INTRODUCTION

Instrument Description

The DM

502

Digital Multimeter measures dc and ac

voltage and current,

dBm, dBV, resistance and

temperature. The

ac

functions are average responding

and

rms

calibrated for sine waves. A single front panel

control selects all ranges. A

front-panel pushbutton

selects dB

readout

of ac

functions in lieu of voltage or

current. Rear interface-connector input is also

pushbut-

ton selected

from the front

panel. Values

of

dB

are

obtained by

adding the

selected dB range

value to the

display reading.

Readout in dBm or dBV

is chosen by

an

internal jumper. The unit is shipped

with the

jumper in the

dBm position.

An internal jumper

also permits selection

of fet input

(>

1000

MO)

or

10 MO input impedance on the two

lowest dc-

voltage ranges.

The unit is shipped with

the jumper in the

10 MO position. See

Fig.

1-2.

Temperature measurements are

made using

a

Tektronix P6430 probe

(Tektronix Part Number010-6430-

00)

or other suitable sensing devices. Option 2 in-

struments delete the

temperature measuring capability.

The readout is

a

3-1/2

digit display

using seven-

segment LED's. The decimal point is automatically

positioned

by

the

range-function switch.

Polarity

indica-

tion is automatic. A usable 50% overrange exists in all

measurement modes except ac voltage and

current,

1000

V dc,

and

the 2 A dc current range. In overrange, the

display blinks and the mostsignificant digit is suppressed.

Upon exceeding

the available 50% overrange, a 1 appears

in the most significant digit position. Display readings are

invalid

beyond

this point.

Interlaced bed output

(1,3, 2,

4)

referenced

to the low

input, is available at the rear interface

connector if user

wired.

NOTE

The DM 502 is not directly compatible with the 153

Calculator Interlace.

The

maximum input voltage

is

1 kV, 350 V at the rear

connectors. Thefront-panel HI and LOconnectors may be

floated

1 kV maximum above ground, the rearinputs 350V.

Installation and Removal

The DM 502 is calibrated and

ready

for

use

when

received. It operates in any compartment of

a

TM 500

series power module. See the power module instruction

manual for line voltage requirements and power module

operation. Fig. 1-1 shows

the DM 502 installation and

removal procedure.

Turn the Power Module

oil before inserting the DM

502. Otherwise, arcing may occur

at the rear inter-

lace

connectors, reducing their

useful life and

damage may be done to

the plug-in circuitry.

Check that the DM 502 is

fully

inserted in the

Power

Module. Pull the PWR switch on the Power Module. One or

more characters in the LEDdisplay

should

now be visible.

The Controls, Connectors and Adjustments foldout

page in Section

3

gives a complete description of the front

panel.

REV.

A, OCT. 1974

1-1

Operating

Instructions—

DM 502

Fig.

1-1. DM

502 Installation and removal.

OPERATING

CONSIDERATIONS

Input Connections

Three

binding

posts provide

measurement connec-

tions. The HI and

LO posts are normally used for

all

measurements

except temperature. Normal

measurement

conditions

will be with the

LO terminal ungrounded. A

connection between the

LO and GND posts maybemade

to

reference

the input against DM

502 chassis ground. Use

caution,

as

the

LO terminal is then connected

to earth

ground

through the three-wire power

cord.

False

readings

may be obtained

due to ground loops.

If the INPUT

button is

pushed

in, signal inputs are

made

via the rear interface

connector; if

the button is out, signal

inputs are made via

the

front

panel.

Functions Available at Rear Connector

HI. LO, GROUND, temperature

output at 10 mV per

degree and bed connections are available

at the

rear

interface connector.

The Rear Interface

Connector

Assignments

page in the Service

Section at the rear of this

manual gives data on

the various connections. Only

the

Temp

Out. Temp Gnd and HI and

LO inputs are factory-

wired. Jumper holes are

provided for making the desired

bed output connections.

See the wire locations illustration

in the foldout

pages for the correct circuit board locations.

Simply

insert and solder an insulated

wire strap in the

proper locations. Buffers

are required before the

data

outputs are used to drive external

loads. See Fig.

1-3

for a

typical buffer circuit.

The

DM 502 has a slot between

pins 1 7 and 18. A barrier

in the corresponding

position of the power module

jack

allows

only compatible

plug-ins to be used in that

compartment.

This protects the plug-in,

should specializ-

ed connections

be madeto that compartment.

Consult the

Building

A System section

of

the

power module manual

for

further information.

1-2

Operating Instructions—DM 502

FET Input

jumper

position

x i

e

<

x i

X

<

x

t

s

I

B X
*

X X
* »

• >

• •

* *
« •

°

a

< o

9

Fig.

1-2.

Locations ol dB reference jumper, input impedance jumper,

and

hard wired

C

or F

component or wire

changes.

Sine-wave Response

The DM 502 responds to the average value of a

sinusoidal

current or

voltage The

readout

indicates

the

equivalent rms value The effective

or

rms value of

a

sine

wave is 0.707 times the peak voltage or current. The

average value is 0.636 of the peak value The scale factor of

the DM 502 is 0.636/0 707 or 0.9. Multiply the DM 502

readout by 0.9

to

obtain the average value of

a

sinusoidal

input voltage

or current.

Temperature Measurements

Connect the P6430 (or other sensing device) to the

connector marked TEMP PROBE.

Use

care to align the

connector pins properly. Select calibration in “F

or °C

with the range-function switch. For best accuracy,

calibrate

the individual

instrument

to the probe used. See

the Internal Adjustment Procedure in the service section

of

this manual for the

proper procedure

13

Operating Instructions—DM 502

The Temp

Out

terminals

at the rear

interface

connector

are always connected to the output of the temperature

circuitry. This feature provides continuous temperature

monitoring capabilities in

°C

even while using other

functions of the DM 502. To avoid

a

momentary

scale

chahge when a recorder or other device is connected to

these

terminals avoid

switching through

the °F

position.

The

DM 502 may be

hard wired in either the °C

or

°F

range

independent of the range-function

switch

setting.

T

o

hard wire in the

°

F range, connect

a

jumper

as

shown in

Fig. 1

-2.

To

hard wire

in

°

C

lift either end of R285 as shown

in Fig.

1-2.

MAKING

MEASUREMENTS

With the DM 502

properly installed in

the power

module, allow twenty

minutes warmup time

for operation

to

specified accuracy. When the value

of the quantity

being

measured is unknown, select

the highest range

first.

To

prevent possible damage to switch

contacts by

Decrease the range

setting until the

display blinks

in-

high currents, do not switch

into, out of, or through

dicating

over-range. Then increase

the range switch to the the

current measurement

modes from the other

next higher position.

This method

obtains maximum measurement

modes with

a

high current source, or

resolution. Resolution of the DM 502

is 0.05% of full scale

large inductive or capacitive

load, connected to the

setting, except

temperature, which is

0.1°

. Do

not exceed DM 502

input. This caution does not

apply to

the

maximum voltage

ratings. With the range-function

switching

between individual current ranges up

to

switch in the 1

K

DC

VOLTS or 500 AC

VOLTS position,

the maximum stated

input current, as the

switch

internal damage may result

before overrange is indicated.

contacts are adequately protected

for this operation.

Fig.

1-3. Typical buffer circuit for rear connector data outputs. Up

to 20

mA may be used from

the DM 502 +5 V supply.

DC Voltage

Measurements

Select

an appropriate full range dc

voltage position

with the range-function

switch. Apply the

voltage

to be

measured to the

INPUT binding posts. Observe the

maximum voltage ratings as indicated on

the front panel.

The readout

displays

a

+

if the HI input is

positive with

respect to the LO

input. A

—

is displayed if

the

LO

input is

more positive. With the

input shorted, the display reads

zero. ±

one count.

DC

Current Measurements

Select an

appropriate full scale DC mA

position on the

range-function switch.

Apply the dc

current

to be

measured

to

the INPUT

binding

posts.

A current (electron

flow) into the LO

connector and out of the

HI connector

indicates

+

on display. For

opposite current flow, a

—

will

be displayed.

1-4

REV.

A,

OCT. 1974

Operating Instructions—DM 502

AC Voltage, Current and

Resistance Measurements

Select an

appropriate full scale AC VOLTS, AC mA or

OHMS position on the

range-function switch. Connect the

unknown voltage, current or

resistance to the INPUT

binding posts.

Decibel Measurements

To

use

this feature,

switch

to

an

ac

voltage or current

range and

push the button marked dB on the

front panel.

Connect the voltage or

current source

to

the DM 502. To

arrive at the correct dB

reading, algebraically add the

number on the

outside scale to the display reading. When

the dB switch is depressed, the

decimal point is fixed and

the maximum display reading is

19.99 on all scales.

For example, if the

range-function switch is

on the

2

AC

VOLTS position and the

display reading is +13.98,

then

the correct dB

reading is:

+13.98

-20

00

-

6

02 dB

If the range-function switch is

on the

200

AC mA

position and the

display

reads

-4.04,

then:

-

4.04

120.00

-15.96

dB

Selection of dB readout in dBV

or

dBm

is accomplished

by a jumper

shown in Fig. 1

-2.

The reference

for

d

BV when

measuring ac voltage is one volt

and when measuring

current one

milliampere The reference for dBm

in the

voltage range is one milliwatt dissipated in

6000 or 0.7746

volts: in the current measuring range

the reference is

0.7746 milliamperes. When dBm or dBV is applied to

current measurements, they have no significance

other

than to the reference current as stated

above.

Useful dB

formulas are:

E'

I'

p,

dB

=

20logio-r

i

,

20 logio -r-

and

10 log

t0

-=p

ti

ii

r

i

For calculations substitute the reference voltage or

current listed as the

denominator.

Temperature

Measurements

With the P6430 probe connected to the front

panel

connector

labeled TEMP

PROBE, set

the range-function

switch

to

the °F or

°C

in the TEMP position. Apply the

probe sensor tip to the device being

measured. For

optimum

temperature transfer,

coat

the surface of the

device being measured with silicone grease and apply the

probe tip squarely to the surface. Allow sufficient time

for

the probe tip to

stabilize before

taking a

reading. The time

required depends upon

several factors. Generally, when

the tip is first applied to the device under test, the readings

change rapidly. As the probe tip

temperature

approaches

the temperature of the device under test, the readings

change less rapidly, and finally stabilize. The

readings are

in

°C

or °F, depending on the position of the range-

function switch. See the

P6430

manual for

more

informa-

tion on

temperature measurements and probe

use.

Using a

Transistor as a Temperature-Sensing

Device

Certain npn transistors suchasa 2N2484 can be used

as separate

sensors

in

place of the P6430 probe with little

or no selection of the transistor. Connect the

temperature-

sensing transistor to the DM 502 through the

TEMP

PROBE connector as shown

in Fig.

1

-4.

Accuracy without

recalibration of the DM 502 is within ±8°C to 150°C. If the

Connector

shell

Connector

TEKTRONIX

Pari Number

131-1653-00

Fig.

1-4. Schematic diagram of temperature-sensing transistor

connected to probe

connector.

REV.

A, OCT.

1974

1-5

Operating Instructions—

DM 502

measurement to

be

made requires

greater accuracy,

recalibrate

the temperature circuitry or

determine a

correction factor.

Place the sensing device in an

environ-

ment

having

a

known ambient

temperature

Use

any

difference between

the known temperature and the DM

502 readout as a

correction factor throughout the

entire

measurement range.

Accurate Current

Source

The OHMS positions on the range-function switch

provide accurate (typically 0

2%)

constant currents at the

INPUT terminals

See

the Ohmmeter specifications for the

value of the currents and maximum voltages across the

INPUT

terminals for full scale display readings. The

maximum voltage available across the INPUT

terminals

in

the OHMS

position is about 10 volts.

ELECTRICAL

SPECIFICATIONS

Performance

Conditions

The electrical characteristics are

valid only if the DM

502 has been

calibrated

at

an ambient

temperature

between

4

20°

C

and

I

30°

C

and

is

operating at an

ambient

temperature between

4l5°Cand

*40°

C

Warm up time to

specified accuracy is twenty minutes.

Display

The measurement rate is 3.33 per

second. Resolution

on

all ranges

is

0.05% of full scale setting;

except for

temperature, which is

0.1°.

DC VOLTMETER

RANGES:

200 mV. 2 V, 20 V, 200

V. 1000 V.

ACCURACY:

*0.1%

of reading, ±1 count.

COMMON MODE

REJECTION:

SHOO

dB at

dc, 80

dB at 50 or 60 Hz

with

a

1 kO

unbalance.

NORMAL MODE

REJECTION:

S

80 dB at 50 or 60 Hz.

STEP RESPONSE

TIME:

$1/2 S.

INPUT RESISTANCE:

1

0

MO

-

0.25%

(jumper selectable

for

>

1

000

MO on 0.2

V and 2 V

ranges).

MAXIMUM

INPUT

VOLTAGE ON ANY RANGE:

Front Panel: 1 kV.

Rear Connector: 350 V

AC VOLTMETER

RANGES:

Voltage: 200 mV. 2 V. 20 V. 200

V, 500 V.

dB: 40, 20, 0. 20,

40. Reference is dBV (1 V)

or

dBm

(1

mW dissipated in 600 O, 0.7746 V).

ACCURACY:

Voltage:

*0.5% of reading,

'

1 count, for a 40 Hz to

10

kHz sine wave.

i 1 0% of

reading,

±

1 count, for

a

20 Hz to 20

kHz sine

wave.

Usable to

100 kHz Typically <10% down

between 40

mV and

500

V at 100 kHz.

1-6

REV. A. OCT. 1974

Operating Instructions— DM

502

DB:

MEASUREMENT CURRENT AND VOLTAGE:

Range Current

Full Scale

Voltage

Display

Frequency

Maximum

Reading

Range

200 o

1 mA

0

2V

0 to

•20

20 Hz to 20 kHz 0.5 dB

2 kO 1 mA

2

V

20 kO

lOOpA

2

V

0 to 10 20

Hz

to 2

kHz

2 kHz to 20

kHz

0.5

dB

1

0

dB

200

kO 10 pA 2 V

i n t/-« on

20

Hz

to 2

kHz

2

kHz to 7.5 kHz

0.5 dB

i n hd

2

MO

1

pA 2 V

1U lO C\J

20 MO

0.1

p

A 2

V

1

.U

Qt)

2.0 dB7.5 kHz to 20 kHz

RESPONSE TIME:

200 O through 2 MOranges,

-.'1/2

s. 20

MO

range.

<5 s.

COMMON MODE REJECTION:

60 dB at 50 or 60

Hz with

a

1 kO unbalance.

RESPONSE

TIME:

$

5 s.

IMPUT IMPEDANCE:

10 MO

±1%

paralleled by

< 60

pF.

MAXIMUM INPUT VOLTAGE ON ANY RANGE:

Front Panel:

500Vac

rms or

l

56

dBm or

600

V

dc,

not

to

exceed 1 kV dc

plus peak

ac.

Rear

Connector:

350 V dc

plus peak ac or

+50 dBm.

OHMMETER

RANGES:

200

O,

2

kO,

20

kO,

200 kO, 2 MO. 20 MO.

ACCURACY:

200

O range,

0 5%

±1 count.

+0.1

O.

2

kO through

2 MO ranges,

+0.5%, ±1 count.

20

MO range,

1.0%,

+1

count.

MAXIMUM

INPUT VOLTAGE

ON ANY RANGE:

130 V

dc and ac rms indefinitely.

250 V dc and ac rms for one-half

hour.

DC AMMETER

RANGES:

200 pA. 2 mA, 20

fnA,

200 mA. 2 A.

ACCURACY:

±0.2% of reading, ±1 count.

RESPONSE TIME:

<1/2

s.

INPUT

IMPEDANCE:

0.2 V

Q 1

0

Range Setting

except <2.0 kO on the

200 pA range

MAXIMUM

INPUT CURRENT ON ANY RANGE:

Fused

at 2.5

A.

AC

AMMETER

RANGES:

Current:

200 pA, 2 mA, 20 mA, 200 mA, 2 A.

dB:

-40,

-20,

0, 20,

40. Reference

is 1 mA (dbV) or

0.7746 mA (dBm).

REV. A. OCT.

1974

1-7

Operating

Instructions—DM

502

ACCURACY:

Current: ±0.6% of reading, ±1

count

for a 40 Hz

to 10

kHz

sine wave. Usable to 100 kHz.

DB:

Display

Reading

Frequency

Range

Maximum

0 to

+20

20 Hz to 20 kHz 0.5 dB

0

to

-10

20

Hz

to 2

kHz 0.5 dB

2 kHz to 20 kHz

1.0

dB

-10to-20

20 Hz to 2 kHz 0.5 dB

2 kHz to 7.5 kHz 1.0 dB

7.5

kHz

to 20 kHz

2.0

dB

RESPONSE TIME:

s.

INPUT IMPEDANCE:

Range

Setting

except < 2.0 KQ on the 200 /jA range.

MAXIMUM INPUT CURRENT ON ANY RANGE:

Fused at 2.5 A.

THERMOMETER

RANGES:

Centrigrade:

—55°

to

+150°.

Fahrenheit:

—67°

to

+200°.

ACCURACY:

With any P6430 probe,

±8°C

or ±14.4°F.

With instrument calibrated to probe. ±2°C or

±3.6°

F.

MAXIMUM

INPUT

VOLTAGE:

Up to

1

kV

may

be

safely applied

to the

front panel

INPUT terminals (350 V to the rear input connectors) when

the range-function switch

is

in

the

TEMP positions.

ENVIRONMENTAL SPECIFICATIONS

TEMPERATURE:

Operating:

+15°C to +40°C.

Non-operating:

—

40° C to +75°C.

POWER CONSUMPTION:

6.5

watts.

1-8

REV. A. OCT.

1974

Section

2—

DM

502

THEORY

OF

OPERATION

THEORY OF

OPERATION

Introduction

The DM 502

is basically an

analog-to-digital converter

with the logic,

display devices, and power supplies

necessary to

display, in digital form,

the value of a

dc

voltage.

The analog-to-digital converter is an integrating

amplifier. Current is

measured

by

passing the unknown

current through a known resistance

and measuring the

resultant voltage drop, using a four

terminal configura-

tion. Resistance is

measured

by

passing a constant

current through an

unknown resistance and measuring

the voltage drop

across

the

unknown resistance.

Temperature is

measured

by

obtaining a dc

voltage

proportional

to

the temperature

and measuring that

voltage.

Ac voltages and currents are rectified and

the

resulting dc is measured,

then read

out

in appropriate ac

units. The dB circuitry converts the output

of the ac

rectifier to a

logarithmic output that is applied to the

analog-to-digital converter. Refer to the

Block Diagram in

the foldout pages for an

overall view of the DM 502

operation.

Use

the following

circuit

description

with the

schematics to gain an

understanding of the operation of

the DM 502.

Attenuator and

Input Switching

The HI and

LO

binding posts are connected

across

R147,

R146 and R145 serving as a voltage divider. The

various

capacitors switched across these resistors

are

the

compensating

capacitors

used

in the

ac

modes of opera-

tion. Cl

00

is switched in series with the divider

when

the

unit is operating in the ac voltage

modes.

R105,

R106, R107,

R110 and R142 serve

as

current

shunts in the current modes. A four terminal

technique is

used.

The current

passes

through the shunts from the HI

terminal to the LO terminal. The

voltageisapplied

to

the

analog-to-digital converter through

S1

10. 17.

16,9,

15 and

18

to ground. FI

00.

in

series with shunts R

105, R106, R107

and

R1

10

along with CR1

10

and CR1

12,

protect the shunts

from over current. The diodes conduct, opening the fuse

before

the

voltage across the higher value shunts damage

them.

Lamp

RT100 protects shunt R142, in the lowest

current range, and the Ohms Converter.

Ac

Converter

In the

ac

measurement modes, the ac signal is

applied

to fet 0165. CR165 and

CR166 protect Q165 from over

voltage.

0170,

connected from the source to the drain of

Q165,

provides bootstrapping.

The

output

of Q165 is fed

to

the negative input of

U195,

an operational

amplifier. A small amount of negative dc

feedback is provided

by

R192, R 190 and the filtering

action

of

C190.

The gain

of

U195

is extremely high until CR187

and CR195 conduct.

This high

gain allows

rectification

of

extremely

small

signals.

More negative

dc

feedback is

provided through the action of CR187

and CR195. The

dc

feedback stabilizes the

operational amplifier gain and

compensates for

amplifier offset voltages or capacitor

leakage.

The positive half cycle is applied through CR195 to the

filter network.

The

dc

voltage from

the

filter

output

is

switched to the analog-to-digital converter input in

the

ac

measurement mode, and the

dB converter

in the dB mode.

The gain

of

U195 is set by

R180 and R181 to produce a

dc

output equivalent

to

the rms value of

a

sine-wave input.

DB Converter

This circuitry converts the linear dc current output from

the

ac

converter to

a

logarithmic output voltage. A

collector current change with respect to a base to emitter

voltage

change

in a

bipolar transistor is logarithmic.

VR310 serves as the voltage

reference diode

at

6.2

V.

This

voltage

is applied

to

the negative input of operational

amplifier

U210

through R203

and

R205. R205 sets

the

current through Q120A. Should the current through

Q120A increase, the voltage at pin 2

of

U210 goes

negative.

This

causes

the

output

of

U210 to go

positive

reducing the current through the transistorto the value set

by

R205. Q120A and Q120B are housed in the same case

so that their base to

emitter voltages remain constant with

temperature (for equal collector currents).

REV.

A. OCT.

1974

21

Theory

of Operation—DM 502

The rectified and filtered dc current from the ac

converter

is

applied to

the plus input of operational

amplifier U220, through R212. R212 causes a reference

current shift in the

dBm

mode

and is

shorted out

in the dBV

mode.

As pin

3

of

U220

goes positive, the output goes

positive. The

base

of Q120B goes positive and the

collector negative

providing negative

feedback.

The

collector current of Q120A always equals

the current

through R21 2 at

0

dB. The necessary voltage at the output

of

U220 is

logarithmic due

to

the logarithmic

characteristics of Q120B in the feedback loop As the

temperature increases the gain of Q120B decreases This

is compensated for by

RT220 decreasing in

value

with

increasing temperature.

Ohms Converter

U135

is an operational amplifier. The current setting

resistances

are

connected

from

the output

to the plus

input terminal of

U135.

The unknown resistances are

connected from the plus terminal to

floating ground.

See

Fig.

2-1. Negative feedback

occurs

from

the

output of

U135 to the

negative input terminal. R

130

is the calibration

adjustment.

R135

and R136 are switched in and out of the

negative feedback loop to provide bias current compensa-

tion.

The value of the current setting resistance

is always

equal to one

half

of the

full scale measurement value,

except on the 200 O range, as shown on

the range-

function switch. The

200 Q scale uses 1 kfi and the gain of

the analog-to-digital converter is increased

by a

factor of

ten. R130 adjusts the gain of the amplifier

to

produce one

volt across the current setting resistance

(0.1

V

in

the

200

Q range). As long

as the

unknown

resistance is within the

measurement range, the circuitry adjusts the current so

that the drop

across the

current

setting

resistance

is one

volt.

This

current flows through the unknown resistance,

varying the voltage

at the

plus terminal of

U135.

This

voltage is then

fed

to the analog-to-digital converter input.

Temperature Measurement Circuitry

This circuitry operates on the principle that

a

change in

base to emitter

voltage of

a

bipolar

transistor is propor-

tional

to

the temperature of the junctionasthe collector

current

is changed

between

two

levels.

A

square

wave

(about 2 kHz) is generated by Q230 and

Q235 operating as an astable multivibrator. The square

wave is

applied

to the gates of Q255 and Q260.

The collector to base voltage of the temperature

sensing transistor in the probe is held constant. When fet

Q255 is off, the current to the probe through R255 and

R257

is approximately one-tenth the on current.

Fig.

2-1. Simplified

diagram of Ohms

Converter.

2-2 REV.

A, OCT.

1974