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

A novel capacitance to digital converter (CDC), which estimates the capa-citance 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 with alternating current excitation and employs 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
$\pm$
220 pF with a nominal value of 400 pF and worst case full scale (F.S) nonlinearity error of
$< $
0.17% is observed. The prototype of CDC is tested for leakage resistance varying from 10 M
$\Omega$
to 50 k
$\Omega$
and worst case error of 1.1% is observed. The prototype of CDC is also tested for stray capacitance, varying from 10 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.