SECTION 4 THEORY OF OPERATION
The temperature sensor is thermistor based. The thermistor sensor is a negative
temperature coefficient device. This means that as the temperature increases,
the resistance of the thermistor decreases. This relationship is a logarithmic func-
tion. The current that flows through the sensor is inversely proportional to the
resistance of the sensor (Ohm’s Law).
U104B and associated resistors minimize the “on” resistance of the gates of
U101A and B. The input signal must be conditioned by the Log Ratio amplifier,
before being digitized by the A/D converter. The Log Ratio amplifier operates on
two phases: the signal phase and the zero phase.
Each phase has a duration equal to
1
⁄2 of the backplane period of the A/D
converter (U102).
Figure 3-2 Model 866 Control Layout
4
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