SECTION 6
Tfwory of Opsnztion
Table 6-l. Range Relays
2Oj.h
K104’
K107 K108
2%k4 KlO7’
2onA
K109
2nA 1 None
:igure 6-2.
Simplified Model
of
Input Amplifier
For example, with the ZmA range sekted, RF = U-2
11
1oom. The oldput voltage with a 1mA input c!uRent is
was = 2cKlpv):
Vou7 =-knAwS2 I IlOO! +200@
= 49898V
Zero Check
Figure 6-3 shows the configuration of the input amplifier
with zero check enabled. With zero check enabled, relay
switch KllO opens forcing the input current to common
throughRZ26
(lOOld2). With the input of U142 open, only
amplifier offset is seen at the output (VOW&
With zero check disabled (relay switch KllO closed), the
input current is simply applied to the (-) input of the am-
plifier as previously shown in Figure 6-Z.
Offset Voltage Adjust
The previous simplified drawings of the input amplifier
showthe(+)input connected tocommon.Achully,a 1OzL
Figure 6-3.
Zero Check ConFguration
resistor is located between the (+) input and common as
shown in Figure 6-4. This allows a trimpot CR1981 to be
used as a coarse adjustment for input offset voltage. Any
remaining offset is canceled when the operator performs
zero correction from the front panel or over the bus.
The offset voltage trimpot is set at the factory and should
not be adjusted unless a component in that circuit is re-
placed. Refer to the troubleshooting information in para-
graph 7.7 ifit becomes necessary to adjust offset voltage.
Offset
Voltage
Adjust
R220
1OCl
-lov
Fipre 6-4.
Offset Voltage Adjust Circuit
6.2.2
Analog Filter
The analog filter of the instrument is shown within the
dashed lines of Figure 6-5. Filtering is provided by
switching in various resistcr/capadtor (RC) combina-
tions.SwitchingisprovidedbyanalogswitchesA,B,C,D
and E.
Table 6-Z identifies the configuration (open or closed) of
each analog switch for all the current ranges. The analog
6-2
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