A Bio-Impedance Readout IC With Digital-Assisted Baseline Cancellation for Two-Electrode Measurement

Abstract

The measurement of the tissue or bio-impedance (BIOZ) is a safe and power-efficient sensing modality that can be adopted for the acquisition of vital signals, such as respiration and heartbeat. A BIOZ readout IC with a wide-input impedance range is proposed. The IC supports vital signal acquisition through a two-electrode setup which requires a larger dynamic range than the conventional four-electrode setup. A digital-assisted baseline impedance cancellation method is implemented to measure small tissue impedance variations originating from respiration and heartbeat in the presence of larger baseline impedances. The proposed technique also mitigates the input-dependent noise behavior of the readout front-end by minimizing the effective input signal to the instrumentation amplifier (IA). The baseline cancellation loop further improves the noise performance by cancelling the reference current noise from the current generator (CG). Hence, one single solution—baseline cancellation—resolves two issues facilitating two-electrode BIOZ setups achieving similar impedance resolution performance compared with more four-electrode setups. The IC, fabricated in a 55-nm CMOS, can measure tissue impedances over a frequency range from 1 kHz to 1 MHz and can achieve a maximum input range of 24 $\text{k}\Omega $ (at the impedance measurement frequency) and a best-case resolution of 2 $\text{m}\Omega $ RMS (for a 100- $\Omega $ input). This increases to 14.0 $\text{m}\Omega $ RMS for a 2- $\text{k}\Omega $ input impedance. The ASIC consumes 18.9–34.9 and 31.4–154.7 $\mu \text{W}$ for the readout front-end and the CG, respectively. The power depends on the injected current amplitudes. A successful demonstration on the human body through a two-electrode (gel and dry) setup confirms the effectivity of the proposed work in a real use case.