A $\Delta \Sigma$ Interface for MEMS Accelerometers Using Electrostatic Spring Constant Modulation for Cancellation of Bondwire Capacitance Drift

This paper presents a closed-loop accelerometer interface based on a force-feedback ΔΣ loop. The interface reduces the offset, and its drift, arising from asymmetry in the parasitic capacitances of the bondwires connecting the CMOS interface IC and the MEMS sensor element. This is achieved by modulating the spring constant of the sensor electrostatically and continuously measuring and nulling the offset via an offset 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 interface was fabricated in a 0.18-μm CMOS technology. The active circuit area is 1.35 mm² and the interface consumes 3.1 mW from a 3-V supply, while achieving a noise floor of 220 μg/√Hz over a 1-kHz bandwidth. When the offset cancellation loop is turned on the bandwidth available for the signal is 200 Hz.