An Oscillator-Based Active Bridge Circuit for Interfacing Capacitive Sensors With Microcontroller Compatibility

This paper deals with the development of a signal conditioning circuit, with microcontroller compatibility, for interfacing capacitive sensors suitable for a wide range of applications. The circuit comprises of a capacitive active bridge, along with a relaxation oscillator for converting capacitance changes into frequency directly. The design, analysis, and experimental results of the circuit and its application to a thin-film-based humidity sensor and a capacitive transducer for measurement of the dielectric constant of edible oil are reported. The experimental results confirm the theoretical values predicted. The frequency output of the circuit has been used to determine the response characteristics of the trace moisture sensor and the dielectric constants of the edible oils. Finally, the output frequency has been calibrated in terms of the trace moisture in parts per million (ppm), using commercial dew point meter (SHAW, U.K.). The accuracy of moisture measurement using custom designed capacitive sensor for the full-scale moisture range of 4-100 ppm, compared with the dew point meter (accuracy ±0.1%), is found to be nearly ±1% and the resolution is 1 ppm moisture. The circuit is less complex and is easy to integrate to application-specific integrated circuits in a standard CMOS technology with high sensitivity (selectable). It can be easily reproduced with the cheapest possible components, while achieving fairly good performance in terms of both accuracy and dynamic range.