A Novel Direct Digitizer for Leaky Differential Capacitive Sensors Using Phase Sensitive Integration

A novel Capacitance to Digital Convertor (CDC) which estimates the capacitance of a leaky Differential Capacitive Sensor (DCS) based on Phase Sensitive Integration (PSI) is presented in this letter. Various methods to measure the capacitance of a leaky capacitive sensor exist, but all these methods apply to single-element capacitive sensor. Most of the techniques which measure capacitance of a DCS are sensitive to leakage resistance of the sensor. The proposed CDC works wi th AC excitation and employs phase sensitive integration (PSI) along with dual slope operation to estimate differential capacitance independent of the leakage resistance. In addition, the proposed CDC is insensitive to the stray capacitance (due to sensor structure or connecting cables) seen at the sensor terminals. Even though the measurand varies in a linear or nonlinear fashion with respect to sensor characterestics, the ratiometric output of the proposed CDC ensures that the output is linear with respect to the measurand. The prototype of CDC is tested for a wide differential range of ± 220 pF with a nominal value of 400 pF and worst case F.S. non-linearity error of >0.17 % is observed. The prototype of CDC is tested for leakage resistance varying from 10 MO to 50 kO and worst case error of 1.1% is observed. The prototype of CDC is also tested for stray capacitance, varying from 10 pF to 200 pF and worst case error observed is 0.39%. Thus the proposed CDC can accurately measure differential capacitance even if leakage resistance is present.