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INSTRUCTION
MANUAL
Digital
Mullimeler
Modds
172A.
173A
THEORY
OF OPERATION
4)
A/D Signal
looks
at
the
bottom
of
RX for
200 milliseconds.
The A/D thus
measure
the
difference
between
A/D Signal
and AZ-2 without
polarity
sign.
The voltage
across
Rx is
displayed.
C.
The measurement
is essentially
a ratio
measurement
between
the
voltage
across
Rx and
the
voltage
across
Rn.
d.
llote
that
the
voltage
at
the
top
of
RX is essentially
at
signal
LO of
the
instrument
(+- nAMP offset
and lead
resistance
drop)
and the
voltage
at
the
bottom
of
Rx is
negative.
Thus,
the
high
impedance
terminal
is guarded
and relative
fast
response
is achieved
at
high
resistance
values,
5)
Refer
to sheet
I of
Schematic
29145E
for
the
remainder
of
the
ohms converter
circuit
discussion.
The ohms switching
FET QIIO
is driven
by the
C-line,
QlO5
is
driven
from
the
A-line
and QlO7
is driven
from
the
B-line.
These
lines
are
also
used
and decoded
to operate
the
Modell73A
autoranging
current.
When on,
QIIO
switches
the
gain
of
UIOI
from
one
to
ten.
QlO5 and QlO7 switch-in
the
range
resistors.
Each
resistor
consists
of
an adjustment
potentiometer,
a fixed
resistor
and
the
on resistance
of
a FET (no
FET for
33.4Mn).
Amplifier
LIZ02 is a FET input
amp1 ifier.
This
amplifier
is overload
protected
by QlOb base
emitter
junction
connected
to signal
ground,
and QlO8 base
emitter
junction
connected
through
QIOV
clamp
transistor.
Thus,
the
input
swing
is
clamped.
Very
large
voltage
excursions
on the
output
side
of
U202 are
allowed
(by
action
of
Q205 and CR205.
Under
norma I
operating
conditions
Q205 is operated
in
the
saturated
mode.
However,
under
overlo,
conditions,
Q2O5 becomes
a current
source
with
its
collector
to emitter
breakdown
handling
large
voltage
excursions
when signals
are
positive
with
respect
to
the
ohms source
high.
Conversely,
if
signals
are
negative
with
respect
to ohms source
high,
CR205 reverse
biases,
cutting
off
that
portion
of
the
circuit.
Al\
of
these
high
voltages
will
cause
current
to
flow
through
R205 (120KQ)
which
is
connected
to
the
+15 volt
supply.
The maximum
limitation
of
250 volts
runs is a function
of
the
power
rating
of
R205 (a l/2-watt
resistor),
and ~205's
400 volt
breakdown.
TABLE 5-4.
OHMS RANGES
I
RANGE
RANGE
REF
OHMS
RANGE LINES
HIR
Lon
R
VI
CURRENT
A
E!
C
3%
3000
3.34K
3.34'/
I mA
tav
- I2V
+Ev
3K
3.34K
.334v
IO!IUA
+8v
-12v
-12v
300K
30K
334 K
3.34v
IOuA
-12v
+8v
+8v
3%
300K
334 K
334V
Iv.4
-12v
+av
-12v
3M
33.4M
;.34v
lOOnA
- 12v
- I2V
+3v
300M
3OM
33.4M
.334'/
IOnA
-12v
-12v
-12v
9.
Clamp
Circuit
(Figure
5-9).
1)
The clamp
prevents
damage
to the
input
circuitry
of
the
A/D converter
and
the
active
filter
by limiting
the
maximum
input
voltage
to approximately
plus
and minus
5 volts.
It
also
functions
in conjunction
with
the
150 Kohm resistor
(R217)
and 0.3ltii
capacitor
(C209)
in
the
dc attenuator
to eliminate
any
relay
arcing
whenever
high
volt-
age
inputs
are
applied
to the
instrument
during
range
switching.
Low leakage
FETS are
used
to minimize
leakage
current.
5-l
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