A readout circuit for wireless passive resonant-circuit sensors

This paper presents a readout circuit for wireless passive LC sensors, where the phase of the readout coil shows a small "dip" nearby the sensor\´s resonant frequency in the low-coupling-coefficient condition (a distance between the readout coil and the LC tank longer than 100 mm). To address this issue and enhance device sensitivity, in this study, a new symmetrical differential drive circuit was proposed where theoretical analysis and experimental verification were performed. The symmetrical differential drive circuit amplified the phase-dip at least 6 times when the scale factor between the reference inductor and the readout coil was 1.138. The proposed readout circuit was used to detect the variations of pressure with a 12 mm×12 mm×14 mm LC pressure sensor at an operational distance up to 100 mm between the LC sensor and the readout circuit, recording a measurement resolution lower than 0.4 kPa.