A Bio-Impedance Readout IC With Complex-Domain Noise-Correlated Baseline Cancellation

This article presents a bio-impedance (bioZ) readout IC featuring a complex-domain noise-correlated baseline cancellation to overcome the limitation of the conventional baseline cancellation with the real-domain noise correlation. The proposed technique is especially beneficial in cases with significant phase shifts between the excitation current from the current generator (CG) and the input voltage signal produced across the bioZ, through which the CG current flows. This technique employs a tunable reference impedance (TRI) and adaptively adjusts it to match the bioZ in both magnitude and phase, thereby achieving a complex-domain correlation of CG noise. By flowing an identical CG current through both the TRI and bioZ, the noise voltages across the TRI and bioZ caused by the CG current become closely correlated with each other, even in the presence of substantial phase shifts, enabling effective CG noise removal after baseline subtraction. Furthermore, this work proposes a differential-difference current-balancing instrumentation amplifier (DD-CBIA) with quiet chopping for baseline subtraction, offering low power consumption, wide input range, and low input-dependent noise. Measurement results demonstrate significant enhancements in noise performance by a factor of 2.47 and 4.88 for the bioZs with phases of 30° and 60°, respectively, achieving a signal-to-noise ratio (SNR) of 101.5 dB and a figure of merit (FoM) of 150.0 dB. Validation through human-subject experiments using two-electrode configurations on the chest and wrist further supports the effectiveness of the proposed bioZ readout IC.