Capacitance measurement circuit for sense-only capacitive sensors operated in mode of measuring a displacement current caused by the presence of a grounded object

摘要

A capacitance measurement circuit for determining a capacitance of a guard-sense capacitive sensor includes a microcontroller that uses a combination of several synchronized PWM outputs for generating a low distortion sine wave. The sine wave is used as a guard voltage for the guard electrode of the capacitive sensor. The capacitance value of an unknown capacitor is measured by impinging the guard voltage on the sense electrode of the capacitive sensor by a modified sigma-delta modulator unit. The digital output of the sigma-delta modulator unit is first multiplied by an XOR gate before being routed into a decimator/low pass filter. The second input of the XOR gate is driven by a square wave from a square wave generator with the same frequency as the guard voltage, but with a substantially 90° phase shift. The output of the decimator/low pass filter is indicative of the capacitance value of the unknown capacitor.