Fluke 845AR User manual

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1

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845AR

High

Impedence

Voltmeter

Null

Detector

Instruction

Manual

WARRANTY

Notwithstanding

any

provision

of

any

agreement

the

following

warranty

is

exclusive:

The

JOHN

FLUKE

MFG.

CO.,

INC.,

warrants

each

instrument

it

manufactures

to

be

free

from

defects

in

material

and

workmanship

under

normal

use

and

service

for

the

period

of

1-year

from

date

of

purchase.

This

warranty

extends

only

to

the

origina!

purchaser.

This

warranty

shall

not

apply

to

fuses,

disposable

batteries

(rechargeable

type

batteries

are

warranted

for

90-days),

or

any

product

or

parts

which

have been

subject

to

misuse,

neglect,

accident,

or

abnormal

conditions

of

operations.

In

the

event

of

failure

of

a

product

covered

by

this

warranty,

John

Fluke

Mfg.

Co.,

Inc.,

will

repair

and

calibrate

an

instrument

returned

to

an

authorized

Service

Facility

within

1

year

of

the

original

purchase;

provided

the

warrantor's

examination

discloses

to

its

satisfaction

that

the

product

was

defective.

The

warrantor

may,

atits

option,

replace

the

product

in

lieu

of

repair.

With

regard

to

any

instrument

returned

within

1

year

of

the

original

purchase,

said

repairs

or

replacement

will

be

made

without

charge.

If

the

failure

has

been

caused

by

misuse,

neglect,

accident,

or

abnormal

conditions

of

operations,

repairs

will

be

billed

at

a

nominal

cost.

In

such

case,

an

estimate

will

be

submitted

before

work

is

started,

if

requested.

THE

FOREGOING

WARRANTY

IS

IN

LIEU

OF

ALL

OTHER

WARRANTIES,

EXPRESS

OR

IMPLIED,

INCLUDING

BUT

NOT

LIMITED

TO

ANY

IMPLIED

WARRANTY

OF

MERCHANTABILITY,

FITNESS,

OR

ADEQUACY

FOR ANY

PARTICULAR

PURPOSE

OR

USE.

JOHN

FLUKE

MFG.

CO.,

INC.,

SHALL

NOT

BE

LIABLE

FOR

ANY

SPECIAL,

INCIDENTAL,

OR

CONSEQUENTIAL

DAMAGES,

WHETHER

IN

CONTRACT,

TORT,

OR

OTHERWISE.

li

any

failure

occurs,

the

foilowing

steps

should

be

taken:

4.

Notify

the

JOHN

FLUKE

MFG.

CO.,

INC.,

or

nearest Service

facility,

giving

full

details

of

the

difficulty,

and

include

the

model

number,

type

number,

and

serial

number.

On

receipt

of

this

information,

service

data,

or

shipping

instructions

will

be

forwarded

to

you.

2.

On

receipt

of

the

shipping

instructions,

forward

the

instrument,

transportation

prepaid.

Repairs

will

be

made

at

the

Service

Facility

and

the

instrument

returned,

transportation

prepaid.

SHIPPING

TO

MANUFACTURER

FOR

REPAIR

OR

ADJUSTMENT

All

shipments

of

JOHN

FLUKE

MFG.

CO.,

INC.,

instruments

should

be

made

via

United

Parcel

Service

or

“Best

Way”

prepaid.

The

instrument

should

be

shipped

in

the

original

packing

carton;

or

if

it

is

not

available,

use

any

suitable

container

that

is

rigid

and

of

adequate

size.

If

a

substitute

container

is

used,

the

instrument

should

be

wrapped

in

paper

and

surrounded

with

at

least

four

inches

of

excelsior

or

similar

shock-absorbing

material.

CLAIM

FOR

DAMAGE

IN

SHIPMENT

TO

ORIGINAL

PURCHASER

The

instrument

should

be

thoroughly

inspected

immediately

upon

original

delivery

to

purchaser.

All

material

in

the

container

should

be

checked

against

the

enclosed

packing

list.

The

manufacturer

will

not

be

responsible

for

shortages

against

the

packing

sheet

unless

notified

immediately.

If

the

instrument

is

damaged

in

any

way,

a

claim

should

be

filed

with

the

carrier

immediately.

(To

obtain

a

quotation

to

repair

shipment

damage,

contact

the

nearest

Fiuke

Technical

Center.)

Final

claim

and

negotiations

with

the

carrier

must

be

completed

by

the

customer.

The

JOHN

FLUKE

MFG.

CO.,

INC,

will

be

happy

to

answer

all

applications

or

use

questions,

which

will

enhance

your

use

of

this

instrument.

Please

address

your

requests

or

correspondence

to:

JOHN

FLUKE

MFG.

CO.,

INC.,

P.O.

BOX

43210,

MOUNTLAKE

TERRACE,

WASHINGTON

98043,

ATTN:

Sales

Dept.

For

European

Customers:

Fluke

(Holland)

B.V.,

P.O.

Box

5053,

5004

EB,

Tilburg,

The

Netherlands.

*For

European

customers,

Air

Freight

prepaid.

John

Fluke

Mfg.

Co.,

Inc.,

P.O.

Box

43210,

Mountlake

Terrace,

Washington

98043

Rev.

4/80

845AR

SECTION

I

INTRODUCTION

AND

SPECIFICATIONS

1-1.

INTRODUCTION

1-2.

The

Fluke

Model

845AR

High

Impedance

Volt-

meter-Null

Detector

allows

measurement

of

de

voltages

from

one

microvolt

to

1000

volts

de

in

19

ranges.

When

used

as

a

null

detector

on

the

100

millivolt

range

and

below,

the

input

impedance

is

an

excellent

19

megohms.™

A

linear

recorder

output

allows

the

instrument

to

be

used

for

production

testing,

and

also

as

a

de

amplifier

with

a

maximum

gain

of

120

db.

1-3.

The

instrument

may

be

wired

to

operate

from

a

line

power

source

of

115

volts

ac

or

230

volts

ac,

as

desired.

The

instrument

is

designed

to

be

mounted

directly

in

a

standard

EIA

19

inch

relay

rack.

Resilient

feet

are

also

provided

for

bench

top

use.

1-4.

ELECTRICAL

SPECIFICATIONS

INPUT

VOLTAGE

RANGE

1

microvolt

to

1000

volts

de

end

scale

in

nineteen

ranges,

using

X1

and

X3

progression.

INPUT

RESISTANCE”

100

megohms

on"300

millivolt

range

and

abovef

10

meg-

Wy

grounding.

Better

than

1010

ohms

up

to

80%

relative

humidity

and

35°C.

With

driven

guard,

isolation

im-

proves

by

at

least

one

order

of

magnitude

up

to

1013

ohms.

Any

input

terminal

may

be

floated

1100

volts

off

chassis

ground.

iy

DC

COMMON

MODE

REJECTION

‘¢Better

than

160

db,

input

short-circuited,

80%

relative

humidity;

better

than

140

db,

open-circuited,

50%

rela-

tive

humidity;

better

than

120

db,

open-circuited,

80%

relative

humidity.

AC

COMMON

MODE

REJECTION

(below

100

kHz)

100

volts

rms

or

120

db

greater

than

end

scale,

which-

ever

is

less,

will

effect

reading

less

than

2%

of

end

scale,

Input

open-circuited.

AC

NORMAL

MODE

REJECTION

(60

Hz

and

above)

AC

voltages

60

db

above

end

scale

will

effect

reading

less

than

2%

of

end

scale.

Maximum

voltage

not

to

ex-

ceed

750

volts

rms.

RECORDER

OUTPUT

0-1

volt,

one

side

at

chassis

ground;

linear

to

0.

5%

of

end

scale,

Source

impedance,

5k

to

7,

5k.

ohms

on

100-millivolt

raage-and-below,

Vesbuy

RASE

JOO

WAKO

Fame

|

Mea

ghwi

ow

VWeSTABILITY

OF

ZERO

ACCURACY

=}

mith

voll

anda,

avb

Yeko.

+(3%

end

scale

+

0.1

microvolt),

MAXIMUM

NOISE

(input

shorted)

Range

Noise

(peak-peak)

0.

20

microvolt

0.25

microvolt

0.30

microvolt

1

microvolt

3

microvolt

10

microvolt

-

1000

volt

METER

RESPONSE

TIME

(to

90%

of

reading)

Range

Time

1

microvolt

5

seconds

3

microvolt

3

seconds

10

microvolt

-

1000

volt

1-1/2

seconds

INPUT

ISOLATION

Better

than

1012

ohms

at

less

than

50%

relative

humidity

and

25°C

regardless

of

line,

chassis,

or

recorder

Better

than

0.15

microvolt/hr,

better

than

0.3

micro-

NOx

hajg

pYout/day.

TEMPERATURE

COEFFICIENT

OF

ZERO

Less

than

0.1

microvolt/°C

from

15°C

to

35°C.

Less

than

0,2

microvolt/°C

from

0°C

to

50°C.

ZERO

CONTROL

RANGE

+5

microvolt

minimum.

OVERLOAD

PROTECTION

Up

to

1100

volts

de

may

be

applied

on

any

range.

Typical

recovery

time

is

4

seconds.

INPUT

POWER

115/230

volts

ac

410%,

50

to

440

Hz,

approximately

3

watts.

1-5.

ENVIRONMENTAL

SPECIFICATIONS

OPERATING

TEMPERATURE

RANGE

Within

all

specifications

from

15°C

to

35°C,

t-I

845AR

Within

all

specifications

from

0°

to

50°C

except:

Derate

by

a

factor

of

two

—

:

Maximum

Noise

and

Meter

Response

Time.

DC

Common

Mode

Rejection

—

Derate

by

20

db.

STORAGE

TEMPERATURE

RANGE

-40°C

to

+70°C.

RELATIVE

HUMIDITY

RANGE

0

to

80%.

SHOCK

VIBRATION

Meets

10

Hz

to

55

Hz

tests

of

MIL-T-945A.

1-6.

MECHANICAL

SPECIFICATIONS

MOUNTING

Standard

EIA

relay

rack.

Resilient

feet

provided

for

bench

use.

WEIGHT

9

pounds,

Meets

hammer

blow

requirements

of

MIL-T-945A

and

SIZE

MIL-~S-901B.

3.47

inches

high

x

19

inches

wide

x

8.26

inches

deep.

19,00"

14,875"

Figure

1-1.

MODEL

845AR

OUTLINE

DRAWING

static

awareness

A

Message

From

John

Fluke

Mig.

Co.,

Inc.

Some

semiconductors

and

custom

IC's

can

be

damaged

by

electrostatic

discharge

during

handling.

This

notice

explains

how

you

can

minimize

the

chances

of

destroying

such

devices

by:

1.

Knowing

that

there

is

a

problem

2.

Learning

the

guidelines

for

handling

them.

3.

Using

the

procedures,

and

packaging

and

bench

techniques

that

are

recommended.

The

Static

Sensitive

(S.S.)

devices

are

identifiedin

the

Fluke

technical

manual

parts

list

with

the

symbol

The

following

practices

should

be

followed

to

minimize

damage

to

S.S.

devices.

3.

DISCHARGE

PERSONAL

STATIC

BEFORE

HANDLING

DEVICES

1.

MINIMIZE

HANDLING

2.

KEEP

PARTS

IN

ORIGINAL

CONTAINERS

UNTIL

READY

FOR

USE.

4.

HANDLE

5.5.

DEVICES

BY

THE

BODY

Page

1

of

2

5.

USE

ANTI-STATIC

CONTAINERS

FOR

HANDLING

AND

TRANSPORT

7.

AVOID

PLASTIC,

VINYL

AND

STYROFOAM®

IN

WORK

AREA

PORTIONS

REPRINTED

WITH

PERMISSION

FROM

TEKTRONIX,

INC.

AND

GENERAL

DYNAMICS,

POMONA

DIV.

@

Dow

Chemical

Page

2

of

2

WHEN

REMOVING

PLUG-IN

ASSEMBLIES,

HANDLE

ONLY

BY

NON-CONDUCTIVE

EDGES

AND

NEVER

TOUCH

OPEN

EDGE

CONNECTOR

EXCEPT

AT

STATIC-FREE

WORK

STATION.

PLACING

SHORTING

STRIPS

ON

EDGE

CONNECTOR

USUALLY

PROVIDES

COMPLETE

PROTECTION

TO

INSTALLED

SS

DEVICES

HANDL

S.

DEVICES

ONLY

AT

A

STATIC-FREE

WORK

STATION

.

ONLY

ANTI-STATIC

TYPE

SOLDER-

SUCKERS

SHOULD

BE

USED.

ONLY

GROUNDED

TIP

SOLDERING

IRONS

SHOULD

BE

USED.

Anti-static

bags,

for

storing

S.S.

devices

or

pcbs

with

these

devices

on

them,

can

be

ordered

from

the

John

Fluke

Mfg.

Co.,

Ine..

See

section

5in

any

Fluke

technical

manual

for

ordering

instructions.

Use

the

following

pari

numbers

when

ordering

these

special

bags.

John

Fluke

Part

No.

Description

463622

6”

X

8"

Bag

453530

8"

X

12”

Bag

453548

16”

X

24”

Bag

454025

12”

X

16"

Bag

Pink

Poly

Sheet

Wrist

Strap

30”x60"x60

Mii

P/N

TL6-60

P/N

RC-AS-1200

$7.00

$20.00

J0089B-07U781G0/SE

EN

Litho

in

U.S.A.

845AR

SECTION

Ii

OPERATING

INSTRUCTIONS

2-1,

RECEIVING

INSPECTION

2-5,

PRELIMINARY

OPERATION

2-2.

This

instrument

has

been

thoroughly

tested

and

2-6,

Connect

the

Model

845AR

line

plug

to

a

115

volt

inspected

before

being

shipped

from

the

factory.

Im-

ac

power

outlet

or

to

230

volts

ac

if

the

instrument

is

so

mediately

upon

receiving

the

instrument,

carefully

in-

wired,

spect

for

damage

which

may

have

occurred

during

ship-

ment,

If

any

damage

is

noted,

follow

the

instructions

|

WARNING!

|

tlined

in

th

he

back

of

thi

A

outlined

in

the

warranty

page

at

the

back

of

this

manual

The

round

pin

on

the

polarized

three-prong

2-3.

CONTROLS,

TERMINALS,

AND

INDICATOR

plug

connects

the

instrument

case

to

power

system

ground.

Use

a

three-to-two

pin

2-4,

The

location

and

function

of

the

front-panel

adapter

when

connecting

to

a

two-contact

controls

are

described

in

Figure

2-1.

Detailed

oper-

outlet.

For

personnel

safety,

connect

the

ating

descriptions

are

given

in

the

following

para-

short

lead

from

the

adapter

to

a

high-quality

graphs.

earth

ground.

METER

;

PR

ZERO

CONTROLS

Indicates

the

voltage

applied

ERO

~

Opens

input

terminals

ISOLATED

OUTPUT

i

nd

shorts

amplifier

input

allow-

LEVEL

CONTROL

Sax

ng

zeroing

of

the

instrument

===

For

full-scale

meter

INPUT

AND

ith

the

lower

zero

control.

POWER

LIGHT

deflection,

this

control

COMMON

TERMINALS

PR

=

Instrument

is

ready

for

Indicates

line

allows

adjustment

of

th

Provide

connection

to

vol-

peration

as

a

voltmeter

or

power

is

applied

—

isolated

output

voltage

tage

being

measured.

ull

detector.

to

the

instrument

from

0

to

1

volt

de,

HH

Saree

CALAS

GUARD

TERMINAL

S

MECHANICAL

ZERO

This

terminal

is

usually

strapped

to

the

common

terminal

Screwdriver

adjustment

used

The

terminal

is

connected

directly

to

an

inner

chassis

o

set

the

meter

to

zero.

ovide

connection

to

shield.

By

removing

the

strap,

a

guard

potential

equal

an

external

recorder

to

the

common

terminal

potential

may

be

applied

to

the

guard

terminal,

shunting

the

leakage

current

from

the

ANGE

CONTROL

common

terminal

to

ground,

Provides

selection

of

the

desired

full-scale

sensitivity.

Figure

2-1,

CONTROLS,

TERMINALS,

AND

INDICATOR

2-1

845AR

a.

Place

the

Model

845AR

controls

as

follows:

POWER

ON

RANGE

10

MICROVOLTS

OPR/ZERO

ZERO

b.

Adjust

thezero

control

for

an

initial

zero

meter

deflection.

Place

the

RANGE

switch

to

the

1

MICROVOLT

RANGE

and

re-zero

with

the

zero

control,

2-7,

MECHANICAL

ZEROING

2-8,

It

may

become

necéssary

to

adjust

the

mechani-

cal

zero

control

of

the

Model

845AR

at

more

frequent

intervals

than

complete

calibration.

To

mechanically

zero

the

instrument

proceed

as

follows:

a.

Place

the

RANGE

switch

to

1000

VOLTS

and

the

POWER

switch

to

ON.

b.

Adjust

the

mechanical

zero

adjustment

screw

for

zero

meter

deflection.

c.

Place

the

RANGE

switch

to

10

MICROVOLTS

and

electrically

zero

the

instrument

as

outlined

in

paragraph

2-5.

d.

Repeat

steps

a

and

b.

2-9.

OPERATION

AS

A

HIGH

IMPEDANCE

VOLTMETER

2-10.

To

operate

the

Model

845AR

as

a

High

Impedance

Voltmeter

perform

the

preliminary

operations

according

to

paragraph

2-5

and

proceed

as

follows:

a.

Place

the

controls

as

follows:

POWER

ON

OPR/ZERO

OPR

RANGE

1000

VOLTS

Mote!

When

measuring

voltages

in

the

microvolt

ranges,

use

copper

wire

having

low

thermal

EMF's.

b.

Connect

the

voltage

to

be

measured

to

the

Model

845AR

INPUT

terminal

and

connect

the

common

point

of

the

voltage

being

measured

to

the

COMMON

terminal.

c.

Deflection

of

the

meter

indicates

the

polarity

and

magnitude

of

the

measured

voltage.

Increase

the

sensitivity

of

the

Model

845AR

for

maximum

on-

scale

deflection.

2-11,

OPERATION

AS

A

NULL

DETECTOR

2-12.

The

Model

845AR

may

be

used

to

monitor

small

voltage

differences

in

bridge

circuits,

potentiometers,

and

other

measuring

apparatus.

In

most

of

these

appli-

cations

the

circuits

are

adjusted

for

zero

deflection

or

a

null

on

the

Model

845AR.

Equipment

connections

for

2-2

various

types

of

null

detector

configurations

are

illus-

trated

by

Figure

2-2

through

2-4.

To

operate

the

Model

845AR

as

a

Null

Detector,

perform

the

preliminary

operations

according

to

paragraph

2-5

and

proceed

as

follows:

a.

Select

the

desired

equipment

application

as

illus-

trated

by

Figure

2-2

through

2-4

and

make

the

appropriate

equipment

connections.

b.

'

Place

the

Model

845AR

controls

as

follows:

POWER

ON

OPR/ZERO

OPR

RANGE

as

desired

c.

Adjust

the

circuit

being

measured

for

zero

or

a

null

deflection

on

the

Model

845AR

meter.

Figure

2-2.

BRIDGE

DETECTOR

-

FLOATING

SUPPLY

Figure

2-3.

BRIDGE

DETECTOR

-

HIGH

RESISTANCE

Figure

2-4,

BRIDGE

DETECTOR

~-

FLOATING

NULL

DETECTOR

2-13.

MEASURING

VOLTAGES

WITH

A

STANDARD

CELL

2-14.

The

Model

845AR

may

be

used

with

a

voltage

divider

and

a

standard

cell

to

calculate

unknown

volt-

ages

with

a

high

degree

of

accuracy.

Connect

the

equip-

ment

as

illustrated

in

Figure

2-5.

Perform

the

pre-

liminary

operation

as

outlined

in

paragraph

2-5

and

proceed

as

follows:

a.

Place

the

Model

845AR

controls

as

follows:

POWER

ON

OPR/ZERO

OPR

RANGE

as

desired

b.

Adjust

the

voltage

divider

for

zero

or

null

deflection

on

the

Model

845AR

meter

while

placing

the

RANGE

switch

to

successively

more

sensitive

ranges.

Figure

2-5.

STANDARD

CELL

VOLTAGE

MEASUREMENTS

845AR

c.

Calculate

the

unknown

voltage

by

dividing

the

standard

cell

voltage

by

the

final

division

ratio

of

the

divider.

2-15.

USE

OF

ISOLATED

OUTPUT

2-16.

DC

ISOLATION

AMPLIFIER

2-17.

The

Model

845AR

may

be

used

as

a

dc

isolation

amplifier

having

a

voltage

gain

of

up

to

120

db,

de~-

pending

on

the

settings

of

the

RANGE

switch

and

the

OUTPUT

LEVEL

control.

To

compute

the

maximum

voltage

gain

on

any

range

of

the

Model

845AR,

use

the

following

formula:

1

volt

(maximum

isolated

output)

Voltage

gain

in

db

=

20

log

i

Range

(in

volts)

2-18.

RECORDER

OUTPUT

2-19..

The

Model

845AR

ISOLATED

OUTPUT

may

be

used

to

provide

an

output

voltage,

adjustable

from

zero

to

one

volt

for

a

full-scale

meter

deflection

for

use

with

a

recorder.

Since

the

output

is

isolated

from

the

input,

floating

measurements

can

be

made

without

the

use

of

a

floating

recorder.

To

use

the

adjustable

recorder

out-

put,

proceed

as

follows:

a,

Connect

the

recorder

to

the

ISOLATED

OUTPUT

terminals.

Note!

The

lower

ISOLATED

OUTPUT

terminal

is

connected

to

chassis

ground.

If

a

ground

reference

is

undesirable,

remove

the

jumper

wire

above

R202

on

the

power

supply

circuit

board.

Refer

to

Figure

2-6

for

jumper

wire

location.

b.

Turn

the

recorder

on.

c.

Proceed

as

outlined

in

paragraph

2-9

or

2-11,

as

desired.

d.

Adjust

the

ISOLATED

OUTPUT

LEVEL

control

for

the

desired

output

to

the

recorder.

This

control

has

a

log

taper

so

that

smooth

control

is

possible

at

both

high

and

low

settings.

Note!

The

ISOLATED

OUTPUT

current

capability

is

100

microamperes

with

a

5

kilohm

source

impedance.

2-20.

OPERATING

NOTES

2-21.

SPURIOUS

VOLTAGES

AND

CURRENTS

2-22,

Voltage

measurements

at

the

microvolt

level

involve

the

persistant

problems

of

thermoelectric

ef-

fects.

These

effects

may

be

compensated

for

by

tempo-

rarily

disconnecting

the

voltage

from

the

circuit

under

measurement

and

noting

the

meter

deflection

of

the

2-3

B45AR

se

tA

eRe

NARADA

near

Model

845AR

on

the

desired

r

then

be

subtracted

from

all

sub:

ments.

A

thorough

underst:

lead

to

reducing

or

eliminating

2-23,

THERMOELECTRIC

VOLTA

2-24.

Wa

circuit

is

composed

of

twa

d

a

net

voltage

will

result

if

iwo

dissi

are

maintained

at

different

temperaiur

moelectric

voltages,

also

known

couple

voltages,

or

Seebeck

ve

by

using

metals

having

iow

thermoele

and

keeping

all

junctions

af

the

same

tem

terminals

of

the

Model

84548

are

made

of

gold-flashed

to

prevent

tarnish.

voltages,

all

connections

te

the

Modsi

$

made

with

pure

copper

wires.

Silver

solder

coated

copper

also

produce

sath

Timed

copper

is

less

satisfactory

than

si

copper

coated

copper.

Nickel

and

ni

are

not

suitable

for

connections

t

Excellent

results

can

be

cbtaine

twin

lead,

or

even

lamp

cord

if

}

sistance

is

not

required.

Hf

shiciding

a

length

of

flat

braid

over

the

cable.

For

2-25.

HIGH

SOURCE

IMPEDANCE

2-26.

Due

to

the

very

high

input

sensitive.

Thus,

a

perso

static

voltage,

can

caus

input

to

the

instrument

and

r

flection

as

a

hand

app

ches

the

input

t

Careful

shielding

will

due

to

charges

that

may

be

nals

when

the

OPR-ZE

appreciable

transi

to

OPR

if

nothing

is

“connected

t

t

Turning

the

switch

back

and

charge,

eliminating

the

prob

impedances,

the

response

of

the

ably

slow

due

to

the

low

pass

superimposed

noise.

Howe

pass

filter

is

such

that

commo:

tremely

high

while

the

response

t

encountered

low

source

impedance:

2-4

»

Also,

input

rI

IRO,

an

GROUND

«

JUMPER

R

WIRE

LOCATION

OVERLOAD

VOLTAGES

The

instrument

is

designed

to

withstand

up

to

1100

volts

peak

ac

continuously

applied

the

three

ingut

terminals

or

between

gr

ound

and

any

of

the

three

input

terminals

tting

of

the

RANGE

or

OPR-ZERO

er,

repeated

or

continuous

overloads

nthe

ranges

below

3

millivolts

will

ion

in

protective,

low-pass-filter

is

will

result

in

thermal

voltages

several

minutes

to

subside

after

the

oved,

ent

hag

an

imer

chassis

connected

inal

on

the

front

panel.

Ordinarily,

ninal

is

strapped

to

the

COMMON

termi-

nnected

in

this

way

the

inner

chassis

id,

This

greatly

improves

the

leakage

ground

and

the

common

mode

rejection.

the

inner

chassis

is

available

at

the

t

may

be

driven

at

the

same

voltage

ION

terminal,

This

further

increases

the

nce

and

common

mode

rejection

by

about

he

voltage

used

to

drive

the

GUARD

termi-

should

be

obtained

from

a

separate

source

or

by

fa

voltage

divider

connected

directly

across

»

drops

across

impedances

in

the

circuit

under

meagurement.

2-31,

INCREASING

INPUT

RESISTANCE

2-88,

Inthe

1

microvolt

to

1

millivolt

ranges,

a

40

megohim

resietor

is

connected

directly

across

the

input

of

the

instrument.

The

input

resistance

may

Hin

in-

creased

oa

these

ranges

by

disconnecting

the

46

meg-

ohm

resistor

where

it

attaches

to

the

RANGE

switch.

However,

the

input

resistance

will

no

longer

be

well

defined.

Typical

input

resistances

with

the

1)

imegohm

noved

are

as

follows:

`

Input

Resistance

300

megohms

1,000

megohms

3,000

megohms

10,000

megohms

PAY

Riles

f

|

j

7

j

Í