A switched-capacitor interface for intelligent capacitive transducers

A novel switched-capacitor circuit has been developed for interfacing capacitive transducers with digital systems. It consists of a differential integrator and a cyclic analog-to-digital converter (ADC). The capacitive transducer is first charged in proportion to its capacitance. This charge is next compared with that stored on a reference capacitor and their difference is converted to a voltage by a differential integrator. The sensitivity of this capacitance-to-voltage conversion is controlled by the rate of the charge accumulation. A binary output is obtained from the resultant voltage by analog-to-digital conversion. The whole operation is insensitive to parasitic capacitances, offset voltages of op amps, and the capacitor values involved in the circuit. Thus the proposed circuit permits an accurate differential capacitance measurement. An error analysis has shown that the resolution as high as 14 bits can be expected by realizing the circuit in a monolithic IC form. Besides the accuracy, it features the small device count integrable onto a small chip area. The circuit is thus suited particularly for the on-chip interface. Its application to a humidity sensor is also presented.