A 0.5mΩ/√Hz 106dB SNR 0.45cm2 Dry-Electrode Bioimpedance Interface with Current Mismatch Cancellation and Boosted Input Impedance of 100MΩ at 50kHz

Abstract

Bioimpedance (BioZ) analysis has been recognized as a new paradigm to derive a number of body composition and hemodynamic measures in a non-invasive manner. Measuring the changes in electrical resistance of the thorax during a cardiac cycle, known as impedance cardiography (ICG), is beneficial in detecting early signs of heart failure deterioration [1]. This raises the need for power-efficient wearable BioZ sensors to enable long-term and user-friendly health monitoring, while state-of-the-art designs still suffer from a few drawbacks. First, conventional BioZ interfaces typically rely on gel electrodes (> 10cm2) for low-impedance contact between skin and electrodes [1]–[2]. This not only hampers the long-term recording but also causes user discomfort. However, it is very challenging to perform small-size dry-electrode BioZ sensing at the frequency range of 1kHz to 1MHz, where both the increased electrode-tissue impedance (ETI) (-10MΩII0.5nF) and input parasitic capacitance $\mathrm{C}_{\mathrm{p}}(> 10\text{pF})$ play a dominating role in attenuating the input signal (Fig. 20.1.1), resulting in gain inaccuracy and a long settling time [3]. To solve this issue, a BioZ amplifier with calibrated positive feedback [3] was proposed for input impedance boosting, which enables 1cm2 dry-electrode BioZ sensing. Second, to identify small BioZ variation (0.01 to $1\Omega$) over higher baseline impedance, i.e., ETI plus the static BioZ component, both the excitation current generator (CG) and the BioZ amplifier must feature low noise. Although dynamic element matching (DEM) is effective in alleviating the 1/f current noise of the CG [2], the input-signal-dependent noise of the amplifier remains a notable problem that severely reduces the measurement accuracy when the BioZ signal is large [4]. Furthermore, previous CGs employing complementary current sources suffer from current mismatch, resulting in low output impedance and limited voltage headroom [1]–[2]. A unipolar CG solves this problem, but both the sinusoidal CG and current sink amplifier are power-hungry [3]. Finally, state-of-the-art BioZ readouts [1]–[3] still require an extra electrode to bias the body and provide the input common-mode (CM) voltage, which further increases the system complexity.