A Novel Capacitance-to-Digital Converter for Capacitive Sensors with AC Excitation

摘要

This paper proposes a novel, simple and high accuracy Capacitance-to-Digital Converter (CDC). Capacitive sensors have numerous industrial applications such as humidity sensing, human touch sensing, flow measurement, ice detection, etc. In most of these applications a sinusoidal excitation is employed or preferred to achieve high sensitivity and accuracy. In order to get a final digital output from a conventional bridge based circuit with a capacitive sensor, an Analog-to-Digital Converter ADC is required. CDCs wherein capacitive sensor is an integral part of a sigma-delta ADC are available but they do not employ a sinusoidal excitation and hence may not give the best output for some of the above mentioned applications. This paper presents a new dual-slope CDC that is designed to accept output from a capacitive sensor, which is excited electrically by a sinusoidal source, and provides a direct digital output proportional to change in capacitance. Such a CDC, having a distinct combination of sinusoidal excitation and dual-slope principle, is best suited for various applications as it provides high accuracy, sensitivity, immunity-to-noise and interference, etc. A prototype CDC has been developed and tested in the laboratory. Experimental results showed the practicality of the proposed scheme.