A Bias-Electrode-Free Multichannel ExG Readout IC With Time-Multiplexed PAM-4 Body Channel Communication

We propose a multi-channel ExG recording system integrated with body channel communication (BCC), targeting wearable multimodal human-machine interfaces, such as extended reality (XR) and brain-computer interfaces (BCIs). To suppress noise from the BCC transmitter that appears as common-mode interference (CMI) at the ExG analog front end (AFE), we use time-multiplexed acquisition scheme that allocates separate time slots for ExG recording and BCC transmission. This temporal separation preserves ultra-low-noise ExG acquisition, enabling reliable recording of EEG and EOG in addition to EMG and ECG. Because the BCC transmitter must transmit multi-channel ExG data within a limited slot, it requires a high data rate. Using PAM-4 signaling, the proposed system achieves a 10-Mbps data rate. This modulation scheme is enabled by a bias-electrode-free ExG AFE, which increases the BCC signal amplitude by approximately 2.13$\boldsymbol{\times}$, thereby providing sufficient amplitude margin for reliable PAM-4 level discrimination. In addition, a charge-pump-based CMI cancellation loop incorporating a multi-channel least-mean-square (LMS) filter, together with a DC servo loop, mitigates inter-channel mismatch and suppresses differential-mode artifacts. Fabricated in 180-nm CMOS, the chips achieve a total common-mode rejection ratio (CMRR) of 101.1 dB and suppress electrode DC offsets up to 500 mV, while maintaining stable operation under 18-Vpp common-mode interference and 15% electrode mismatch.