A Large-Area LTPS-TFT-Based Bi-directional Biomedical Interface with Process-Invariant In-pixel Biopotential-to-Digital Converters

In this paper, we demonstrate a bi-directional biomedical process-invariant pixel interface based on the LTPS-TFT that integrates a front-end amplifier, an analog-to-digital converter and a stimulator. The pixel interface can convert biopotential into digital signals. This near-sensor signal processing can avoid the interference of motion artifacts, and the all-digital transfer has a high noise tolerance. Under the process variation of 1×-1.4× threshold voltage and ±10% mobility fluctuations, the DC gain of the operational amplifier changes from 58.83 dB to 57.21 dB, and 39.09 dB to 37.88 dB for the front-end amplifier. Compared with the single-stage amplifier, the stability has been improved by 13.2×. The proposed pseudo differential VCO-based ADC can effectively eliminate second-order non-linearity which achieves the best performance in state-of-the-art TFT-ADCs. When the threshold voltage and mobility fluctuate, ENOB changes from 7.36 bit to 7.30 bit (OSR=64) and 11.52 bit to 11.14 bit (OSR=256). The change rate does not exceed 4%.