An active CMOS LPF for MEMS capacitive accelerometer readout

This paper presents an active low-pass filter with its bandwidth ranging from 100Hz to 8kHz. The signals lower than 500Hz are filtered through the switched capacitor low-pass filter, while those higher than 500Hz are filtered through the continuous-time one. Problems such as charge injection and clock feedthrough in the switched-capacitor circuit are also analyzed and worked out in this design. We use the optimal switch sequence to help eliminate the nonlinearity caused by the charge injection, dummy switches to reduce the coupled charge, and a folded cascode differential input stage with high PSRR to restrain the interference from the power supply. The circuit performance, therefore, has been greatly improved with a half area sharply reduced below 500Hz, especially meeting the needs of MEMS capacitive accelerometer readout.