Autonulling-Based Multichannel Impedance Measurement System for Capacitive Sensors

Capacitive sensors are widely used in a variety of contact and noncontact sensing applications. Low-power consumption and the small size of the capacitive sensors with high sensitivity make them an excellent choice for sensor arrays for the Internet of Things. The conventional capacitance measurement solutions, such as capacitance-to-digital converters FDC1004 from Texas Instruments and AD7746 from Analog Devices, provide excellent resolution but are limited by the number of channels and small measurement range. In this letter, we present a multichannel high-accuracy and wide-range impedance measurement system for capacitive sensors. The proposed system is based on lock-in amplification and autonulling techniques for accurately measuring the sensor capacitance. The proposed system utilized a time-division multiplexing-based approach with a frequency divider network for easy reconfigurable multichannel measurement. A prototype of the proposed system for 16-channel impedance measurement was built and tested. The proposed system was tested for different measurement ranges from 40 to 3000 pF, recording a resolution as low as 0.2 pF for capacitance measurement on all channels.