An Orthogonal Code-Division Multiplexing AFE with low Channel-Gain-Mismatch by Input Capacitors Calibration Technology

An orthogonal code-division multiplexing analog front-end (AFE) for electroencephalogram (EEG) acquisition with low inter-channel gain mismatch is proposed in this paper. By adopting an orthogonal code-division multiplexing scheme based on Walsh-Hadamard codes, the area and power consumption of the circuit are effectively saved and the crosstalk between channels is maximally suppressed. In addition, an input capacitor calibration technique is proposed to improve the match of the capacitors by calibrating the input capacitors of each channel successively. The proposed AFE was fabricated in a 65 nm CMOS process. The measured results show that the gain range is $34-60 \mathrm{~dB}$, and the gain mismatch of the multi-channel AFE is reduced to 0.33%.