An improved capacitance measurement technique based of RC phase delay

Capacitance can be measured by utilizing the phase delay through an RC network consisting of the unknown capacitance and a resistor. Specifically, the RC network delays the state change of an input square wave to produce a pulse width modulated signal based on the phase delay through the RC network. The response of this implementation, however, becomes severely nonlinear if the phase delay is greater than approximately 45°. An improved implementation is presented that avoids the nonlinearity by using a MOSFET switch to discharge the unknown capacitor during each measurement cycle. A prototype circuit was implemented and tested with a 0.7-20pF variable capacitor and a linear response was measured with a sensitivity of 246mV/pF when the output pulse width modulated signal was lowpass filtered, level shifted and amplified.