Designing a High Input-Impedance Buffer for Dry-Electrode Bioimpedance Analysis

Recent advances in wearable sensing platforms and robust, dry electrode materials have enabled adhesive-free, continuous bioimpedance sensing [1]. However, long-term continuous monitoring of bioimpedance requires high-impedance dry-electrodes which can exacerbate input-impedance mismatch errors. We present a modified mathematical model and design methodology for designing a high-input impedance buffer to improve voltage sensing in four-electrode bioimpedance applications. The bootstrapped buffer design presented confers nearly 75% less common-mode to differential-mode conversion at low-frequencies, and 50% less error at high-frequencies based on the worst-case model. The voltage buffer was directly used and alternated with the baseline inputs inputs of a commercially available off the shelf integrated circuit (AD5940) to determine the extent of transient bioimpedance errors emerging from input impedance mismatch. In a small pilot study of two young male adults, we show the system reduces the time for the 5% convergence time of the final bioimpedance by 100.7s seconds on average. The voltage buffer typically reduces bioimpedance errors by 20% compared to the baseline inputs based on exponential decay curves of best-fit in dry-electrode applications.