Temperature-to-frequency converter

Al's positive input is biased by the thermocouple. Al's output drives a crude V — F converter, comprised of the 74C04 inverters and associated components. Each V — F output pulse causes a fixed quantity of charge to be dispensed into the 1 µ¥ capacitor from the 100 pF capacitor via the LT1043 switch. The larger capacitor integrates the packets of charge, producing a dc voltage at Al's negative input. Al's output forces the V-* F converter to run at whatever frequency is required to balance the amplifier's inputs.
Temperature-to-frequency converter  - schematic

This feedback action eliminates drift and nonlinearities in the V — F converter as an error item and the output frequency is solely a function of the dc conditions at Al's inputs. The 3300 pF capacitor forms a dominant response pole at Al, stabilizing the loop.

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