A CAPACITANCE RATIO TO FREQUENCY RATIO CONVERTER USING SWITCHED-CAPACITOR TECHNIQUES

This paper introduces a switched-capacitor circuit for interfacing capacitance ratio sensors to digital instrumentation. This work was motivated by the increasing needs of field bus systems for compact low-power interface circuits. Sensors producing an output proportional to the capacitance ratio are of particular practical importance. The circuit reported here produces a frequency ratio output in response to a capacitance ratio input. Errors resulting from clock feedthrough and charge injection are shown to cause significant deviations from the first-order theory of operation of the circuit when discrete switch components are used. The first-order theory has been extended to include the effect of charge-induced errors and experimental results show close agreement between theory and practice.