High-performance differential capacitive mems sensor readout with relaxation oscillator front-end and phase locked loop time-to-digital converter back-end

Digital oriented front-end and signal processing interface for capacitive MEMS sensor is an attractive alternative to the conventional analog intensive design. The main challenge comes from improving the noise performance of digitally oriented front-end which will dictate the performance of the entire system. We are proposing a new interface that uses an RC relaxation oscillator front-end as a capacitance to duty cycle converter, followed by a phase locked loop time-to-digital converter which provides fine/coarse resolution digital output. Utilizing the RC relaxation oscillator output as the input reference to the phase locked loop, the time-to-digital conversion becomes synchronized with the front-end thus significantly improves the overall system noise performance. From measurement, our design has a flat output noise density profile of −103 dBFS with an Allan Deviation floor of −104 dBFS, non-linearity of ≤0.35 %FS across a wide bandwidth of 0.5 MHz. Our design can be used directly as an interface for any differential capacitive MEMS sensor to achieve high-performance sensing with direct digital output.

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