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HYDRA
Service Manual
2A-26
For the 300V range (Figure 2A-4), the HI signal is again scaled by A3Z4. The input is
applied to pin 1 of A3Z4, and a 1000:1 divider is formed by the 10-MΩ and 10.01-kΩ
resistors when switches S3 and S9 are closed in A3Z4. The attenuated HI input is then
sent through internal switch S10 to the passive filter and the a/d converter. LO is sensed
through analog processor switch S18 and resistor A3R34.
PASSIVE
FILTER
A/D
HIGH
LOW
A3R34
A3Z4
10.01k
A3K16
S9
S3
S10
A3Z4
10M
A3R10
A3R11
A3K17
S2
INPUT HI
INPUT LO
s14f.eps
Figure 2A-4. DC Volts 300V Range Simplified Schematic (2635A)
2A-54. Ohms and RTDs
Resistance measurements are made using a ratio ohms technique, as shown in Figure
2A-5. A stable voltage source is connected in series with the reference resistor in A3Z4
and the unknown resistor. Since the same current flows through both resistors, the
unknown resistance can be determined by multiplying the ratio of the voltage drops
across the reference and the unknown resistors by the known reference resistor value.