An interface circuit for R-C impedance sensors with a relaxation oscillator

A simple interface circuit for impedance sensors based on a relaxation oscillator is presented. The measurement strategy and the principle of operation are discussed. The circuit is intended for measuring impedance, which can be represented as a capacitor and a resistor in series. By means of a four-signal measurement technique, continuous self-calibration is achieved. The output signals for both components of the unknown impedance are time periods, and for calculating their values only one reference capacitor is needed. A measurement set-up is presented with which the new method was tested. The experimental results prove that with this new technique, very high resolution for both components of the measured impedance can be achieved-better than 0.1 Ω for R. and better than 0.1 pF for C_x