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

This paper presents a ring oscillator based readout circuit for differential capacitive MEMS. Flicker noise is reduced by modulating it to frequencies above the bandwidth of interest and then filtering. A technique equivalent to chopping in conventional switched capacitor circuits is developed and applied to a single ended ring oscillator controlled by the MEMS capacitance. A prototype was designed in a 0.12 um CMOS process. Simulations show that flicker noise is attenuated and Signal-to-Noise Ratio is improved.