Filter
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HYDRA
Service Manual
2-22
PASSIVE
FILTER
A/D
HIGH
LOW
A3R34
A3Z4
10.01k
A3K16
S9
S3
S10
A3Z4
10M
A3R10
A3R11
A3K17
S2
INPUT HI
INPUT LO
s4f.eps
Figure 2-4. DC Volts 300V Range Simplified Schematic
When an input is switched in for a measurement, the ohms source in Analog Processor
A3U8 is set to the correct voltage for the range selected and is connected to the
appropriate reference resistor in network A3Z4. A measurement current then flows
through A3Z4, relay A3K16, thermistor A3RT1, resistor A3R10, the unknown
resistance, A3R43, ground, and the ohms source.
The resulting voltage across the unknown resistance is integrated for a fixed period of
time by the A/D Converter through the HI SENSE path of A3R11, A3K17, A3R42 and
A3U8 switch S2, and the LO SENSE path of A3R35 and Analog Processor switch S19.
Passive filtering is provided by A3C34, A3C27, and portions or all of the DC Filter
block.
The voltage across the reference resistor for the 300Ω and RTD, 3-kΩ, and 30-kΩ
ranges (the 1-kΩ, 10.01-kΩ, and 100.5-kΩ resistances in A3Z4, respectively) is
integrated for a variable period of time until the voltage across the integrate capacitor
reaches zero. For the 300Ω and RTD range, the reference resistor voltage is switched in
through Analog Processor switch S6 and applied to the A/D Converter by switch S8. For
the 3-kΩ range, switches S9 and S11 perform these functions, respectively. For the 30-
kΩ range, switches S13 and S14 are used. For all ranges, the voltage is routed through
A3R34 to the RRS input.