An Interface Circuit With Wide Dynamic Range for Differential Capacitive Sensing Applications

An interface circuit for differential capacitive sensing applications with tunable dynamic range is presented. Capacitive microsensors are ubiquitously employed in many applications pertaining to all aspects of modern life. The proposed circuit requires a small chip area and offers a good signal-to-noise ratio as well as adjustable sensitivity and dynamic range. Reference signals are produced on chip and used for the synchronous demodulation of current signals through a differential capacitive sensor. In addition to the normal open-loop operation mode of the circuit, it can be operated within a novel closed-loop configuration in order to extend its dynamic range. The circuit was designed and fabricated in a standard 0.35-mu m CMOS technology from Austriamicrosystems. Experimental and simulation results are presented and discussed. The circuit is capable of resolving 0.4 fF of variation in capacitance with a 50-kHz measurement bandwidth. Reducing the bandwidth to 1 kHz for signal frequencies around 10 kHz increases the dynamic range of the closed-loop circuit to 99 and 110 dB for open-loop and closed-loop circuits, respectively.