Capacitance-to-phase angle conversion for the detection of extremely small capacities

Much progress has been made over the last few years in scaling down the dimensions of transducers. In particular, silicon micromachining technology has allowed small integrated sensors to be developed. The advantage of scaling of integrated capacitive transducers is mainly restricted by the limitations imposed by the capacitance change that can be resolved by the readout circuits. This paper describes a suitable technique for the readout of capacitive transducers using an ac-operated charge amplifier. The method is based on the compensation of the nominal transducer capacitance using a voltage which is slightly less than 180° out of phase. This setup makes it possible to convert the capacitance to a phase angle and to detect capacitance changes smaller than 1 fF.