A Capacitance-to-Digital Converter Based on a Ring Oscillator with Flicker Noise Reduction

This paper presents a ring oscillator based readoutcircuit for differential capacitive MEMS. Flicker noise isreduced by modulating it to frequencies above the bandwidthof interest and then filtering. A technique equivalent tochopping in conventional switched capacitor circuits isdeveloped and applied to a single ended ring oscillatorcontrolled by the MEMS capacitance. A prototype wasdesigned in a 0.12 um CMOS process. Simulations show thatflicker noise is attenuated and Signal-to-Noise Ratio isimproved.