A 97.3-dB SNR Bioimpedance AFE With −84-dB THD Segmented- ΔΣ M Sinusoidal Current Generator and Passing-Through Instrumentation Amplifier

This article presents a high-precision bioimpedance (BioZ) analog front end (AFE) to detect changes in tissue electrical properties as prognostic indicators for heart failure, cancer, and chronic pulmonary disease. To increase the BioZ measurement accuracy, this AFE achieves key technical improvements in three aspects: 1) a lookup table (LUT) with segmented delta-sigma modulator ( $\Delta \Sigma $ M) for area-efficient bit extension of the IDAC to produce low-distortion sinusoidal current; 2) a current amplifier (CA) and a passing-through instrumentation amplifier (PT-IA) to mitigate the 1/f noise modulation in current generator (CG) and readout, respectively; and 3) a $\Delta \Sigma $ M ADC with tri-level dynamic element matching (TL-DEM) IDAC to achieve reference current noise cancellation while maintaining high linearity performance. Manufactured in a 0.18- $\mu $ m CMOS process, the overall AFE consumes 86.7–201.1 and $54.8~{\mu }$ W, respectively, from the CG and readout. The CG achieves −84-dB total harmonic distortion (THD) and 170-pA/ $\surd $ Hz stimulating current at $100~{\mu }$ Apk, while the AFE achieves 97.3-dB system signal-to-noise ratio (SNR) (BW $ = 4$ Hz) and 0.64-m $\Omega $ / $\surd $ Hz sensitivity at 1 Hz. Impedance cardiography (ICG) has been successfully measured with dry electrodes.