Theory
of
Operation
OHMS
PROTECTION
REF
AMP
SUPPLY
2702
1
Figure
56.
Precision Voltage Reference
Large positive input voltages are blocked by CR402. Large
voltage drop across the resistor is measured ("sensa
negative input voltages are dropped equally across three
in the VDC function.
high-voltage transistors
(Q402,
Q403, and Q404). If -300V
is
present
at
the collector of Q404, the voltage drops
The voltage sensed at the INPUT terminals is
sea]
equally across 2402
so
that large negative voltages never
shown by the
switch table in Figure 5-8.
(
reach the current source.
to the track
period
of the trackhold cycle, during
the scaled ihput voltage
is
sampled.)
The circuitry associated with Q408
(R406,
R407, R408,
R409, Q406,
Q408,
and CR403) clamps the open-circuit
In
the lower four
the
full
voltage of the Ohms Current Source below +65V in the
AID
Converter is 2V. However, in the
22000
kS2
a1
lower four ranges and below +13V dc in the higher two
MQ
ranges, the full-scale input voltage to the A/D
ranges. The in-guard PC turns Q408 on or off depending
verter is +1V; the in-guard
WC
completes the scali~
on range. In the lower four ohms ranges,
Q408
is on,
multiplying the
A/D
result by
2.
effectively shorting
R409;
R406 and R403 then form a
519.
wire
ohlns
voltage divider
which
clamps the output of the ohms
In the 4-wife ohms function, the Ohms Current Sou
source
45V-
In the
higher
two
Ohms
connected tothe INPUT HI terminal by ohms relay
ranges, Q408 is off, including R409
in
the voltage divider
in
2-wire
Ohms
(Figurr 5-8).
The
Ohms
Current
and clamping the output below +13V.
applies a known current
to
the resistance unde~
5-17.
OHMS
FUNCTIONS
through the
INPUT
HI and INPUT
LO
leads. The resl
voltage drop across the resistor is measured by the Sl
5-18.
2-Wire
Ohms
HI and SENSE
LO
leads.
In the 2-wire ohms fundion, the Ohms Current Source is
The voltage at the SENSE HI terminal is connected
I
connected to the INPUT HI terminal by ohms relay
K401
DC
Scaling circuit by Q303 (Figure 5-8). The volu
(Figure 5-8). The Ohms Current Source applies
a
known
then
scaled
exactly
as
in the 2-wire ohms function.
(
current to the resistance under test, and the resulting
to
the track period in the switch table in Figure
5-8.)
5-8
the
20
on-
by
:
is
to 1
rce
test
ing
'SE
the
:
is
:fer
3
10
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