Patent ID: 7119555

Claim:
A circuit for detecting a difference in capacitance between a first capacitor and a second capacitor provided in a sensor, comprising: an oscillator configured to generate an oscillating signal; a phase comparator coupled to said oscillator to output a signal responsive to a phase difference between the oscillating signal delayed by said first capacitor and the oscillating signal delayed by said second capacitor; a phase difference converting circuit coupled to said phase comparator to convert the phase difference into an electrical quantity; an integration circuit coupled to said phase difference converting circuit to output an integrated signal made by integrating the electrical quantity over a time period equal to a predetermined number of cycles of the oscillating signal; a sample-and-hold circuit coupled to said integration circuit to output a signal made by sampling and holding the integrated signal at substantially an end of said time period; and a reference signal circuit coupled between said integration circuit and said sample-and-hold circuit to return a level of the integrated signal to a reference level after the integrated signal is sampled and held by said sample-and-hold circuit.