Analysis of the Switched-Capacitor Dual-Slope Capacitance-to-Digital Converter

A dual-slope capacitance-to-digital converter (CDC) that operates on the elements of a differential capacitive sensor and provides a digital output that is linearly proportional to the physical quantity being sensed by the sensor is presented and analyzed in this paper. The converter topology is so chosen that a linear digital output is obtained for not only a sensor possessing linear input-output characteristics but also a sensor possessing inverse characteristics. The digital output in the proposed converter is dependent only on, apart from the sensitivity of the sensor, a dc reference voltage. Hence, high accuracy and linearity are easily obtained by employing a precision dc reference. Since the proposed CDC is based on the popular dual-slope analog-to-digital converter structure, it possesses all the advantages (resolution, accuracy, and immunity to noise and component parameter variations) and limitations (requirement of auto-zero and low conversion speed) applicable to the dual-slope technique. A prototype built and tested for a typical differential capacitive sensor with a nominal capacitance value of 250 pF gave a worst-case error of less than 0.05%.