A Digital Frequency-Locked Loop System for Capacitance Measurement

Similar to phase-locked loops, frequency-locked loops (FLLs) are useful in many applications involving waveform synchronization or synthesis. Simple logic circuit-based relaxation oscillators convert capacitance to frequency, which is a characteristic inverse relationship between output frequency and input capacitance. The oscillator's logic level square-wave output can be fed into an all-digital FLL that will frequency lock to the input signal and produce a digital output wordN, whereNis inversely proportional to the input frequency. The result is thatNis linearly proportional to the unknown capacitance in the oscillator. This novel approach allows a simple FFL implementation for capacitance measurement and is demonstrated in hardware using a capacitive sensor that measures the mass of small quantities of water with an output capacitance range of 75-185 pF.