A novel current scaling active pixel sensor with correlated double sampling readout circuit for real time medical x-ray imaging

This paper presents a new current-programmed, current-output active pixel sensor (APS) based on a novel current scaling scheme and suitable for real time X-ray imaging (fluoroscopy) and a current mode CMOS readout circuit. The circuit is designed using hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) technology. The simulation results based on the measured characteristics of a-Si:H TFTs show that the proposed pixel circuit can successfully scale up the current data during readout and therefore provide higher gain compared with the conventional current mode APS circuits. The design can also 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.