## 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 <inline-formula><tex-math notation="LaTeX">$19.66 \,\mathrm{Hz}$</tex-math></inline-formula>/<inline-formula><tex-math notation="LaTeX">$\Omega$</tex-math></inline-formula> at <inline-formula><tex-math notation="LaTeX">$1.1 \,\mathrm{k}\Omega$</tex-math></inline-formula> load impedance magnitude and a current consumption of <inline-formula><tex-math notation="LaTeX">$128 \,\upmu\mathrm{A}$</tex-math></inline-formula>. As a demonstration we show the application of the I-to-F converter for human gesture recognition and for radial pulse sensing.

Original language | English |
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Pages (from-to) | 1-12 |

Number of pages | 12 |

Journal | IEEE Transactions on Biomedical Circuits and Systems |

DOIs | |

State | Accepted/In press - 2024 |

## Keywords

- Capacitance
- Frequency division multiplexing
- Frequency measurement
- Impedance
- Impedance-to-frequency converter
- Sensors
- Transducers
- Wearable sensors
- frequency multiplexing
- multi-modal sensors
- strain gauge sensor
- wearable sensor