A Interdigitater Extended CMOS-MEMS Capacitive Sensing System with Bypass Successive Approximation Register Capacitance to Digital Converter Readout

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A pressure sensing system consisting of the MEMS pressure sensor and the readout circuits on a single chip by CMOS MEMS technology is presented. In-house wet etching post process is used to release the MEMS structure. Pressure signal is converted into capacitance value by the proposed on-chip MEMS capacitive sensor then digitized by an Bypass Successive-Approximation-Register Capacitance to Digital Converter(Bypass SAR CDC) .Parasitic capacitance, noise and area of sensing system is greatly reduced by the integration of capacitive sensor and CDC on a single chip. The proposed Bypass SAR CDC is composed of a Interdigitater Extended CMOS MEMS Pressure Sensor, a Bypass Comparator and Bypass Control Logic; The sensor parallel plate capacitance structure is combined with interdigitated structure to increase the unit capacitance value, and increase the sensitivity of the sensor using an extended architecture. Place this sensor on a capacitor array in the Bypass SAR CDC. Hence, when the sensing capacitance changes, it will produce voltage drift. Then the system will track the voltage drift and generate output digital codes. In addition, Bypass SAR CDC can automatically adjust the power according to the ICP input signal size to achive power optimization performance. The sensitivity and sensing range of MEMS capacitive sensor are 53.25 pF/Mpa and 0-40kPa, respectively. The prototype chip is fabricated using the UMC 0.18μm 1P6M CMOS technology with an area of 1.5*1.5〖mm〗^2, 0.9μW/1.62μW(Bypass/no Bypass) power consumption and FoM 0.1 7pJ/step (Bypass) 0.307 pJ/step (no Bypass). Keywords: MEMS, capacitive sensor, sensor readout, Bypass SAR CDC