Patent ID: 8604809

Claim:
A capacitance measuring circuit, comprising: an oscillator circuit having: a sensor capacitance which forms part of a series connection of at least two series connected impedances, the sensor capacitance being connected to the capacitance measuring circuit only via end terminals of the series connection, one end terminal being connected to a constant voltage and another end terminal of the series connection being connected to a low impedance sensing node, the oscillator circuit oscillating with a square wave voltage at the low impedance sensing node to maintain constant voltage levels during each half-period of the square wave voltage and simultaneously sensing a charging current flowing into the at least two series connected impedances to enable triggering of the capacitance measuring circuit when a sensed charging current has changed to predetermined levels; a first buffer for generating the square wave voltage, said first buffer having two different supply voltages, an output voltage of the first buffer being connected through a current sensing resistor to the low impedance sensing node and said current sensing resistor being arranged to sense said charging current; and a comparator having a reference level and configured to control the first buffer to switch the output voltage between the different supply voltages when current through the current sensing resistor crosses said reference level.