A 3-TFT hybrid active-passive pixel with correlated double sampling CMOS readout circuit for real-time medical x-ray imaging

This paper presents a new hybrid current-programmed, current-output active pixel sensor (APS) suitable for real time X-ray imaging (fluoroscopy) and an off-panel CMOS readout circuit. The pixel circuit is designed using hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) technology. Measurements show that the proposed pixel circuit can successfully compensate for characteristic variations (e.g. mobility and threshold voltage shift) in a-Si:H TFTs under prolonged gate voltage stress. The readout circuit exploits correlated double sampling (CDS) technique to reduce the offset current, low frequency noise and fixed-pattern noise (FPN) on the array operation. Measurements show less than 5% change in the output current of the APS for 3 V shift in the threshold voltage.