An EMG Interface Comprising a Flexible a-IGZO Active Electrode Matrix and a 65-nm CMOS IC

In this work, a $4\times $ 4 active electrode (AE) matrix integrated on a flexible foil substrate to monitor muscular contraction is presented. The electrodes and their front ends (FEs) (i.e., AEs) are implemented on foil in amorphous indium gallium zinc oxide (a-IGZO) thin-film transistor (TFT) technology and interfaced with a custom 65-nm CMOS chip where analog-to-digital conversion and serial transmission of data from multiple channels are performed. On-foil frequency multiplexing and conversion to current domain are used to transmit to the silicon (Si)-IC, all the signals coming from a column of electrodes via a single output. This strategy enables interfacing the 26-V-supply flexible electronics with the 1.2-V-supply CMOS IC, while reducing the number of interconnects and Si area devoted to pads. The a-IGZO FE includes an integrated switched-capacitor high-pass input filter offering a 25–40- $\text{M}\Omega $ impedance in the band of interest. Each AE on foil occupies $4\times $ 4 mm, enabling electromyography (EMG) measurements with millimeter (mm)-range resolution. The a-IGZO amplifier in the AE, with its intrinsic bandpass response, achieves a process and temperature-insensitive 20-dB in-band gain and a 23-dB dc rejection. The back-end Si-IC can compensate variable bias currents and offsets from the flexible FE exploiting an offset-compensated trans-impedance amplifier (TIA) with 50-dB-dc rejection and negligible noise contribution. The a-IGZO FE alone achieves 20- $\mu \text{A}$ /V gain, 30-kHz bandwidth (BW), 55- $\mu \text{V}_{\mathrm {rms}}$ input-referred noise, consuming 5 mW. The system has been validated in vivo extracting the muscle fiber conduction velocity from the tibialis anterior of a volunteer.