CMOS low noise capacitance measurement circuit for micromachined gyroscope

Abstract

A CMOS low noise, high sensitivity circuit suitable for detecting the change in capacitance along the sensing direction of a vibrating gyroscope with interdigitated finger comb structure is presented. The circuit is used in conjunction with a fully differential capacitor bridge, which is formed by the sensing port of the mechanical structure. A mathematical model of a gyroscope is established for electrical simulation with electrical equivalent circuit. Theoretical analysis and computer simulations using HSPICE show that as low as 0.2aF change in capacitance is detectable under ideal condition. Two high frequency carriers of the same magnitude and frequency are used in the circuit. Simulation results review that the resolution of the technique is severely affected by the mismatch of the high frequency carriers. The effect of parasitic capacitance, which is inherent in the silicon micromachining technology, can be greatly reduced by selecting appropriate circuit configuration.