An ECG Monitoring System Using SAR ADC on Flexible Foil With Oxide TFT Technology

This manuscript presents an electrocardiogram (ECG) monitoring system with charge redistribution noise shaping successive approximation register analog to digital converter (CR-NS SAR ADC). The proposed system has successfully demonstrated an ECG reconstruction while preserving crucial parameters such as PR, QT, ST, QRS, RR interval, and heart rate, which are essential for analyzing ECG data. The ADC is designed using single-gate Indium-Gallium-Zinc-Oxide thin-film transistor (IGZO TFT) technology only with n-type TFTs and fabricated on a 30 μm thick polyimide substrate. Further, this work has addressed circuit design challenges that arise due to only n-type transistor based implementation. This ADC employs an energy efficient modified CR switching scheme, which utilizes merged capacitor switching (MCS) technique. Moreover, for the first time, first-order passive NS technique is experimented with this technology in the ADC. From measurements, this circuit shows a walden figure-of-merit (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FoM_W</i>) of 13.8 nJ/c.s, schreier FoM (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FoM_S</i>) of 98.298 dB, SNR of 39.23 dB and ENOB of 6-bits. The proposed ADC operates up to a sampling frequency of 2.08 kHz with a power consumption of 1.85mW at a supply voltage (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_DD</i>) of 4V. This design is well suited for biomedical wearable devices because of its low-power operation and compatibility with flexible substrates.