A Interface for MEMS Accelerometers Using Electrostatic Spring Constant Modulation for Cancellation of Bondwire

Abstract

This paper presents a closed-loop accelerometer inter- face based on a force-feedback loop. The interface reduces the offset, and its drift, arising from asymmetry in the parasitic capaci- tances of the bondwires connecting the CMOS interface IC and the MEMS sensor element. This is achieved by modulating the spring c o n s t a n to ft h es e n s o re l e c t r o s t a tically and continuously measuring and nulling the offset via an offse t cancellation loop. In a bondwire deformation experiment, we show that the offset cancellation loop reduces an induced offset of 350 mg by 54 dB down to 0.7 mg. The in- terface was fabricated in a 0.18- m CMOS technology. The active circuit area is 1.35 mm and the interface consumes 3.1 mW from a 3-V supply, while achieving a noise floor of 220 over a 1-kHz bandwidth. When the offset cancellation loop is turned on, the bandwidth available for the signal is 200 Hz.