A Low-Power Impedance-to-Frequency Converter for Frequency-Multiplexed Wearable Sensors

Abstract

We propose a low-power impedance-to-frequency (I-to-F) converter for wearable transducers that change both its resistance and capacitance in response to mechanical deformation or changes in ambient pressure. At the core of the proposed I-to-F converter is a fixed-point circuit comprising of a voltage-controlled relaxation oscillator and a proportional-to-temperature (PTAT) current reference that locks the oscillation frequency according to the impedance of the transducer. Using both analytical and measurement results we show that the operation of the proposed I-to-F converter is well matched to a specific class of sponge mechanical transducer where the system can achieve higher sensitivity when compared to a simple resistance measurement techniques. Furthermore, the oscillation frequency of the converter can be programmed to ensure that multiple transducer and I-to-F converters can communicate simultaneously over a shared channel (physical wire or virtual wireless channel) using frequency-division multiplexing. Measured results from proof-of-concept prototypes show an impedance sensitivity of $19.66 \,\mathrm{Hz}$ / $\Omega$ at $1.1 \,\mathrm{k}\Omega$ load impedance magnitude and a current consumption of 128 μ $\mathrm{A}$ . As a demonstration we show the application of the I-to-F converter for human gesture recognition and for radial pulse sensing.