Patent ID: 6700392
Filing Date: 2004-03-02
Classification: G01R,H03K

Abstract:
A circuit for sensing the value of the mutual capacitance between a sensor capacitor and a target, the circuit comprising:a sensing capacitor having a sensing capacitance value, the sensing capacitor having first and second terminals, the second terminal coupled to a voltage reference point; a first amplifier having an input and an output, the input connected to the first terminal of the sensing capacitor, the amplifier providing an output signal that is proportional to the input signal; an input capacitance compensation circuit coupled between the input and output of the first amplifier, the input capacitance compensation circuit operative to reduce the effect of the parasitic input capacitance of the first amplifier; a relaxation oscillator coupled to the first amplifier and receiving the output signal therefrom, the relaxation oscillator including the sensing capacitor and the first amplifier, the relaxation oscillator having an output including first and second values, wherein the output of the relaxation oscillator is the first value for a first relaxation time and the output of the relaxation oscillator is the second value for a second relaxation time, and wherein the sum of the first relaxation time and the second relaxation time is proportional to the sensing capacitance value.