Design and noise analysis of a sigma-delta capacitive micromachined accelerometer

A single-loop fourth-order sigma-delta (σ Δ) interface circuit for a closed-loop micromachined accelerometer is presented. Two additional electronic integrators are cascaded with the micromachined sensing element to form a fourth-order loop filter. The three main noise sources affecting the overall system resolution of a σ Δ accelerometer, mechanical noise, electronic noise and quantization noise, are analyzed in detail. Accurate mathematical formulas for electronic and quantization noise are established. The ASIC is fabricated in a 0.5 μm two-metal two-poly n-well CMOS process. The test results indicate that the mechanical noise and electronic noise are 1 μg/√Hz and 8 μV/√Hz respectively, and the theoretical models of electronic and quantization noise agree well with the test and simulation results.