Datron 1062 - Page 42

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31

The

voltage

developed

across

C8

causes

IVI4

to

sink

current

through

resistor chain R24,

R25, R29,

R30, R31

until the

voltage developed

across

the chain

balances that across C8,

Thus

the current

required

for

a

particular

range is selected

by

the value of

the resistor

chain switched by Ml, M2

and

Q4,

Simplified

diagram

Fig. 3.30

shows the resistor

chain

and

switching for each

range. On the high resistance

ranges

leakage paths

are provided by

Q3,

M2(B) and M2(C).

To

produce good

common mode

rejection, M4

supplies are

bootstrapped, the

supply span being defined by

a 12

volt zener,

D17.

The

filtered bootstrap supplies

(+^2BS

and —OBS)

power the

astable (M6) and

bilateral

switch.

The use

of ohms

guard

permits in-circuit

measurement

of resistors,

provided shunt

paths

are

greater than

25012

and a

suitable

tapping point

is available.

Consider Fig.

3.31

.

Guard

is reference

0,

Lo

is actively

maintained

within

microvolts

of reference

0

(as

previously explained).

Thus

there is no

voltage across

Rz and

consequently no current

in Rz. Voltage

follower

Q10/M3

will simply pass

more

current

into

Ry

from the I-(-

terminal until the

selected

current for the

particular range

flows through

Rx.

3.4.3 Test

During the

self-test routine

(actuated

from the

front

panel or

remotely

programmed),

the

Ohms

Converter is

checked for

correct

operation.

The

circuitry is placed

into

the

10kl2

range as

described

in

Section 3.

2. 1.3.

Filter

is

selected

and

F.E.T. Q5

'closed'

from M9-1

causing R8

(9.76kl2) to be

placed

between

I-t-

and I—.

Thus

with

I-t

and

Hi, I- and Lo

connected

(2-wire if front

panel input

selected),

the

DC

Isolator

(which

is also in the

TEST

mode)

measures the

voltage

developed

across

the resistor

(approx

1

volt). The

resulting voltage

output

from the DC

Isolator

is

applied

to

the A-D

converter,

measured and

compared

to

the

stored

value.

If

the

measured

signal is

within

+

6%

of the

stored value, the test

is complete.

3.5

CURRENT

ASSEMBLY

(Drawing No. 430304)

The Current assembly

contains a set

of

selectable

precision

current

shunts,

the voltage

developed across

the

shunt(s)

being sampled

by

the DC

or AC

voltage

measure-

ment

circuits.