28.5 A 0.6V/0.9V 26.6-to-119.3µW ΔΣ-Based Bio-Impedance Readout IC with 101.9dB SNR and <0.1Hz 1/f Corner

Bio-impedance (BioZ) is an important physiological parameter in wearable healthcare sensing. Besides the inherent cardiac and respiratory information, BioZ can be also used for other emerging applications such as non-invasive blood status sensing [1]. A conventional 4-electrode (4E) setup eliminates the effect of electrode-tissue impedance (ETI) at the expense of user comfort, system complexity, and cost. On the other hand, a 2-electrode (2E) setup avoids short-falls of 4E but can only capture relative changes of BioZ instead of its absolute value. In addition, a readout front-end (RFE) with wide dynamic range (DR) and high signal-to-noise ratio (SNR) is needed to deal with small BioZ variation (0.1~10Ω) as well as large baseline resistance (>10kΩ). A conventional RFE architecture employing an instrumentation amplifier (IA) and ADC has to trade-off between resolution, DR and noise [2,3]. Although flicker noise in the current generator (CG) is mitigated through dynamic element matching (DEM) [2], the reference current (IREF) noise issue remains unaddressed. In [5], digital-assisted baseline cancellation and IREF correlated noise cancellation are proposed, which help eliminate IREF noise and input-dependent noise [4] due to the large signal in the current-balance instrumentation amplifier (CBIA). Nevertheless, larger noise is still observed due to the finite residual current (I
_res
) from the baseline cancellation.