Electronic technique and circuit topology for accurate distance-independent contactless readout of passive LC sensors

Contactless readout of passive LC sensors composed of a capacitance sensor connected to a coil can be performed through a readout coil electromagnetically coupled with the sensor coil. Resonant frequency and quality factor can be extracted from the impedance measured at the readout coil by a technique which results theoretically independent of the coupling, and, therefore, the distance between the readout and the sensor coils. However, the effect of the unavoidable parasitic capacitance connected to the readout coil introduces in practice an undesired dependence of the measured values on the distance between the coils, resulting in a decreased accuracy. In this paper, such dependence is predicted by the results of a numerical analysis and experimentally verified. To overcome this limitation, a novel electronic technique and circuit topology for the compensation of the readout parasitic capacitance is proposed. The experimental results show that the compensation technique allows to read the resonant frequency of a LC pair at around 5.3 MHz with a variation of less than 200 ppm across an interrogation distance between 2 and 24 mm.