reference element is a reference amplifier (ref amp). The
nominal ref amp voltage is 6.5V.
Resistor R701, precision resistor network 2701, and tran-
sistorhener diode combination U701 are produced
as
a
matched set
so
that the output of U702A is precisely
-7.00000V. This output is remotely sensed at the pins of
the custom
A/D
IC (U101). Diode CR701 prevents the
output from going positive at power-up.
U702B functions
as
an inverter to provide the +7.00000V
output and to supply the reference amplifier. The gain of
U702B is set
by
the two 20 kS2 resistors in
the
resistor
network 2702.
5-15. OHMS CURRENT SOURCE
The
Ohms Current Source (Figure
5-7)
provides a precise
test current for the ohms functions. The first stage (U401,
R401, and Q401) produces a precise reference current,
using precision resistor R401
and
a
-7.0000V
reference
voltage from the Precision Voltage Reference.
The second
stage
(U404,
precision resistor networh 2401,
and analog switches
U402
and U403)
is
a current an/lplifier
whose
gain
is controlled by the In-Guard
&.
pe In-
Guard
pC
sets the output current for each range
bly
con-
trolling
U402
and U403. (See switch state table in Figure
5-7.)
5-16. OHMS PROTECTION
The Ohms Protection circuit
(Q402,
Q403,
Q404,
Q405,
Q406,
and Q407) clamps the open circuit voltage of the
Ohms Current Source and provides protection 6r the
Ohms Current Source.
The circuit: protects the Ohms Current Source
froq up to
k300V across the INPUT terminals. The circuit also
clamps voltage transients larger than 1560V with four
MOVs (RV401, RV402, RV403, and RV404). In addition,
a
1
kS2,
2W
fusible wire-wound resistor (R410) in series
with the output current path fails open-circuit under
extreme overvoltage conditions.
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