The Research And Design Of Capacitance To Digital Converter For Pressure Sensors

Pressure sensors based on MEMS technology are widely used in detection of pressure, altitude, acceleration, liquid flow, etc. Piezo-resistive and capacitive MEMS pressure sensor are two common types of pressure sensors. Compared to the piezo-resistive type, capacitive type is less affected by variation in temperature. For ease of use, capacitive pressure sensors that has digital output are in vast demand.There are many methods to detect the capacitance change in capacitive pressure sensors such as capacitor to voltage (C/V), capacitive to time (C/T) etc. The traditional method is to convert the capacitance change to voltage change and then digitize this voltage change using an analog-to-digital converter. However, the voltage output using this method is very much affected by circuit inherent and environmental noise. The problem can be avoided by converting capacitance directly to time. However, the resolution that can be attained using this method is limited by the performance of the digital counter.In this paper, a new approach of converting capacitance to digital signal has been proposed for MEMS pressure sensor. The change in capacitance of the sensor is converted to digital signal using a novel digital bit-increment technique to achieve high resolution and high linearity capacitance-to-digital converter. The conversion process goes through four modules, the capacitance-to-time converter (CTC), the time-to-digital converter (TDC), the digital control module, and finally the digital bit-increment module. Compared to the resolution that present pressure sensors are able to achieve,the improvement in resolution using the technique is substantial. Moreover, system using this technique is less affected by the variation in supply voltage and can accommodate wide input range. The proposed MEMS capacitive pressure sensor readout circuit is realized using standard 0.25?m CMOS technology for a pressure sensor of specifications 100Hz bandwidth, 24pF nominal capacitance value and 8pF full-range capacitance change. The simulation results show a minimum detectable capacitance of 3.9aF, a resolution of 21 bit and a linearity of 3.908ppm are achieved.The chip area is 1.06 mm2...