A Low-Channel-Mismatch ExG AFE Based on Orthogonal Nested-Chopper and Successive-Approximation Input Capacitance Calibration

This article proposes a low-channel-mismatch ExG analog front end (AFE) for biosensors. An orthogonal nested-chopper technique is proposed to suppress the mismatch in interchannel input impedance. The first stage of the orthogonal nested chopper has the same frequency to ensure that each channel has the same input impedance. The second-stage chopper is controlled by Walsh-Hadamard codes, which can suppress the signal crosstalk among channels and lower the system’s modulation frequency. In addition, an input capacitance calibration technique based on successive-approximation (SA) logic is proposed, effectively suppressing the gain mismatch by gradually calibrating the input capacitance of each channel. The proposed AFE was fabricated in a 65-nm CMOS process, and the core area is $0.95\times 0.9$ mm2. The measured results show that the proposed AFE consumes an average of $2.6~\mu $ W per channel at a 1.2-V supply voltage. The input impedance is greater than 1.96 G $\Omega $ , and the mismatch in input impedance among channels is 0.21%. The gain range is 26–46 dB, and the gain mismatch among channels is only 0.11%. The total common-mode rejection ratio (CMRR) is increased to 91 dB. The proposed AFE can clearly acquire electrocardiography (ECG), electromyography (EMG), and electroencephalography (EEG).