A Closed-Loop ΣΔ Interface for a High-Q Micromechanical Capacitive Accelerometer With 200 ng/√Hz Input Noise Density

In this paper, a fully-differential high-order switchedcapacitor (SC) sigma-delta ()Sigma Delta interface in a standard 0.5 mu m CMOS technology for a micromechanical capacitive accelerometer is presented. A 1 bit digital output is attained by the interface circuit based on a low-noise front-end and a back-end third-order SC Sigma Delta modulator, avoiding the use of a separate high-resolution Sigma Delta converter to digitize the analog feedback signal. In addition, a micromechanical capacitive sensor element with a high quality factor (high-Q) is used to achieve high-resolution. Furthermore, closed-loop operation and electrical compensation are employed for damping the high-Q to ensure the stability of the high-order system. The sensor consumes 23 mW of power from a single 7 V supply at a sampling clock of 250 kHz. Meanwhile, a sensitivity of 1.896 Vig is achieved with a noise floor of lower than 200 ng/N/root Hz in low-frequency. The sensor has a nonlinearity of 0.15% with an input range of +/- 1.2g. The bandwidth (BW) is 300 Hz and the bias instability is 18 mu g.