A 0.48$^{\circ}$ Phase Error 91.1 dB SNR Bioimpedance Measurement IC for Monitoring Cardiopulmonary Diseases

This article presents a low-power and low phase error bioimpedance (BioZ) measurement IC designed for monitoring cardiopulmonary diseases. To compensate for the phase shift introduced along the signal path by current generator (CG), electrodes and sensor analog front-end (AFE), a novel phase shift calibration logic is proposed. Utilizing this calibration logic, a single-channel in-phase demodulation-based impedance measurement scheme is developed. A noise shaping pseudo-sine wave CG with data-weighted averaging (DWA) is used to minimize modulation harmonics. Fabricated in a 0.18µm CMOS process, the chip occupies 0.73 mm2 and consumes between 52.7 to 97.5 µA current from a 1.8V supply. The CG achieves 74.1 dB SFDR and -70dB THD at 15.5 kHz with a 50µApk stimulation current. The chip achieves 2mΩ/√Hz input-referred impedance noise at 1Hz, 91.1 dB SNR (BW = 4 Hz), 36 kΩ input range and less than 0.48$^{\circ}$ phase error (0 - 90$^{\circ}$, 1 - 20 kHz). On-body BioZ experiments using a 4-electrode configuration demonstrate clear recordings of Impedance Cardiography (ICG) and respiration signals.