A Technique for Improving the Linear Operating Range for a Relative Phase Delay Capacitive Sensor Interface Circuit

One technique for measuring an unknown sensor capacitance is using the phase delay of an
RC
network with a resistor and the sensor capacitor. By buffering the
RC
network with CMOS inverters, a square wave signal can be delayed through the circuit in proportion to the unknown capacitance. An EXOR gate can be used to compare the delayed square wave with a reference square wave to produce a pulsewidth-modulated signal where the pulsewidth is proportional to the unknown capacitance. This technique has a nearly linear response when the phase delay is small compared with the period of the square wave. However, this technique becomes severely nonlinear if the phase delay is larger than approximately one eighth of the period (45°), as the capacitor does not get fully charged or discharged every half period. To mitigate this nonlinearity, a novel resistance switching
RC
phase delay network is used to expand the linear range of the phase delay by accelerating the charging and discharging processes of the sensor capacitor. This approach is demonstrated using a fringing field capacitive sensor to measure the mass of added water and expands the linear operation to a phase delay to 133.56°.