CAPACITANCE MEASURING CIRCUIT

摘要

In a capacitance measuring circuit 1, comprising an oscillator circuit, the sensor capacitance Ct forms a link of two or more series connected impedances Rt, Ct, which is connected to the capacitance measuring circuit only at the end terminals of the series connection. One terminal of the series connection, which is connected to a constant voltage and the other end terminal of the series connection is connected to a sensing node Cu, which oscillates with a square wave voltage and at the same time senses the charging current flowing into the series connected impedances Rt, Ct, to enable the capacitance measuring circuit to be triggered at the moments when the charging current thus sensed has changed to predetermined levels. The complete capacitance measuring circuit, including the sensing node, is switched alternately between two voltages, whereby the influence of stray capacitances Cs on the oscillator frequency, which is representative of the sensor capacitance is avoided.