A sigma-delta interface ASIC for force-feedback micromachined capacitive accelerometer

A single-loop fourth-order sigma-delta (∑Δ) interface circuit for micromachined accelerometer is presented in this study. Two additional electronic integrators are cascaded with the micromachine sensing element to form a fourth-order loop filter to eliminate quantization noise. A precise model for the overall system is set up based on nonlinear model of 1-bit quantizer. Three main noise sources affecting the overall system resolution of a ∑Δ accelerometer: mechanical noise, electronic noise and quantization noise are analyzed in more detail. A switched-capacitor charge integrator and correlated double sampling are applied to reduce input-referred electronic noise. The ASIC is fabricated in 0.5 μm two-metal two-poly n-well CMOS process, and test results show that the noise density floors of the open-loop and closed-loop modes are 12 and 80 μg/Hz ^1/2, respectively, the sensitivity is 1.25 V/g, the full measurement range can be achieved from -2 to +2 g, and the power dissipation is 40 mW.