Low-cost, high-precision measurement system for capacitive sensors

This paper describes a low-cost realization of a precision capacitance measurement instrument. The method is based on synchronous modulation and demodulation operating at a frequency of 100 kHz. A two-channel multiplying digital-to-analogue converter is used to generate a sinusoidal drive voltage and to perform multiplication with the demodulation signal. The modulation and demodulation signals are generated using a direct digital synthesizer, which is implemented using a field programmable gate array. Experimental results show a reproducibility of better than +/-0.005% in the range from 50 pF to 600 pF.