Abstract:
Operating MEMS capacitive sensors in negative feedback mode results in improved bandwidth, and lower sensitivity to process and temperature variation. Feedback operation is achieved by applying a feedback voltage to the actuation electrodes of the sensor, generating a corresponding feedback-force on the sensor proof mass. To overcome, the non-linearity of the quadratic voltage-to-force relation in capacitive feedback, a two-level voltage feedback signal is often used. Therefore, a singlebit Σ-Δ modulator represents a practical way to implement force-feedback sensors interface systems. However, single-bit Σ-Δ modulators have a limited input-range that is less than the available full-scale dictated by the actuation voltage value. This is caused by quantizer overload, and the consequent reduction in quantizer effective gain as the input signal approaches full-scale. In this work, a solution is proposed that allows extending the input signal range of Σ-Δ based capacitive sensors beyond the limit imposed by single-bit operation. The proposed technique is applied to the design of a MEMS based accelerometer, and results in an increase in the input signal range from 35g to 40g, and an improvement in signal-to-noise ratio from 130.2dB to 137.3dB, at the same actuation voltage level.
