Advanced RC Phase Delay Capacitive Sensor Interface Circuits for MEMS

Abstract

Many types of sensors in MEMS technology convert a measurand to a proportional change in capacitance. One of the techniques of measuring capacitance is utilizing the phase delay of an RC network with a resistor and the sensor capacitor. Specifically, the state of an input square wave signal is delayed by the RC network and gives a pulse width modulated signal according to the phase delay at the output, which is proportional to the capacitance, if the resistance is fixed. However, the response of this method becomes severely nonlinear if the phase delay is bigger than approximately 45°. Two improved implementations are presented to avoid the nonlinearity caused by the capacitor being not fully charged and discharged each cycle. The first one uses a PMOSFET switch to charge the unknown capacitor and an NMOSFET to discharge it during each measurement cycle. The second one uses an analog switch to switch the resistance to be significantly lower when the capacitor needs to be fully charged or discharged in each cycle. Both methods were simulated and proved effective. Prototype circuits were also implemented and tested with an 8-128pF variable capacitor, and linear responses were measured. These techniques produced a linear response over different and large capacitance ranges, and can possess fast response times.