A readout circuit for wireless passive LC sensors and its application for gastrointestinal monitoring

Abstract

A readout circuit is presented for wireless passive LC sensors, where an inductor–capacitor (LC) resonant circuit was combined with a readout coil for resonant frequency detection. The impedance phase of the readout coil shows a ‘dip’ near the sensor’s resonant frequency due to the mutual inductance. Previously, the phase-dip has suffered from limited amplitude in the low-coupling-coefficient condition (especially in the case of implantation), rendering portable detection troublesome. To address this issue, in this study a new differential transduction circuit was proposed where both theoretical analysis and numerical simulations were performed. Compared to conventional transduction circuits (e.g., the I–V circuit and the auto-balancing bridge circuit), the differential circuit was more sensitive to the phase change, enabling more reliable and precise resonant frequency detection. Moreover, the proposed readout circuit was used to detect the gastrointestinal pressure of rabbits with a Ø10 mm × 14 mm LC pressure sensor at an operational distance of up to 60 mm between the LC sensor and the readout circuit. Experimental results recorded a measurement resolution lower than 0.4 kPa and a measurement speed of eight times per second.