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HYDRA
Service Manual
2-26
2-59. A/D Converter
Figure 2-7 shows the dual slope a/d converter used in the instrument. The unknown input
voltage is buffered and used to charge (integrate) a capacitor for an exact period of time.
This integrator capacitor is then discharged by the buffered output of a stable and
accurate reference voltage of opposite polarity. The capacitor discharge time, which is
proportional to the level of the unknown input signal, is measured by the digital circuits
in the Analog Measurement Processor. This time count becomes the conversion result.
BUFFER
A3Z2
INTEGRATOR
INTEGRATE
REFERENCE
INTEGRATE
INPUT
–REFERENCE
(+ INPUT)
REFERENCE
+ REFERENCE
(– INPUT)
S77
A3C13
+
_
_
+
+
_
A/D
COMPARATOR
+
_
COUNTER
INPUT HI
INPUT LO
s7f.eps
Figure 2-7. A/D Converter Simplified Schematic
In both the slow and fast measurement rates, the a/d converter uses its ±300 mV range
for most measurement functions and ranges. The primary exceptions are that the 3V dc
range is measured on the a/d converter 3V range, thermocouples are measured on the
±100 mV range, and the temperature reference is measured on the 1V a/d converter
range. The typical overload point on a slow rate 30000 count range is 32000 display
counts; the typical overload point on a fast rate 3000 count range is 3200 display counts.
During the integrate phase, the a/d buffer in the A3U8 Analog Measurement Processor
applies the signal to be measured to one of the four integrator input resistors in network
A3Z2. As shown on the A/D Converter schematic diagram in Section 8, the choice of
resistor selects the a/d converter range. Switch S69 connects the buffer output through
pin B.1 for the 100-mV range, S71 connects the output through B.32 for the 300 mV
range, S73 connects to pin B1 for the 1V range, and S75 sets up the 3V range through
pin B3.2.