Design of an Interface Circuit for MEMS Capacitive Sensors

Abstract

Integrated Micro-Electro-Mechanical (MEMS) sensors and the signal conditioning circuits on the same chip are gaining popularity in various system applications. A MEMS based sensor mainly consists of a mechanical sensing element requires an interfacing circuit to provide a usable output. Here, we have designed an interfacing circuit for MEMS capacitive accelerometer to convert the acceleration into corresponding electrical voltage. In MEMS capacitive accelerometers the change in capacitance is in the Femto Farad (FF) range. Here, the interface circuit is implement with voltage mode half bridge configuration with differential sensing for measuring the change in capacitance. The interfacing circuit includes a buffer, variable gain amplifier, demodulator to give phase sensitive and low pass filter. The Variable gain amplifier is used for different gain selection and demodulator produces phase sensitive output to give the direction of acceleration change. Low pass filter is used to get stable DC voltage. For the implementation of different circuit blocks, we have designed a Miller compensated two-stage op-amp with DC gain of 61 dB and the phase margin is 52◦ . The sensitivity of the interface circuit is tunable and it depends on the amplitude of square-wave excitation signal and the gain of variable gain amplifier.The complete circuit is designed in UMC 0.18 µm CMOS process technology and the simulation results and layout of various blocks are presented.