An Active-Pixel Readout Circuit Technique towards all LTPS-TFT-on-foil Large-Area Imagers with Inherent Nonlinearity Compensation

An amplifier placed within a pixel is advantageous, as it offers gain at the first stage in the signal readout chain, but is limited by non-linearity. This paper presents a readout-circuit technique, that addresses the non-linearity issue of a current-mode pixel. The proposed circuit named ‘Pixel Follower’ compensates for the non-linearity by inverting it via feedback implemented locally. This avoids having to reduce the in-pixel amplifier gain, or sending any feedback signal to the pixel itself, thus greatly simplifying the design compared to other published techniques. The circuit has been manufactured in a Low Temperature Polycrystalline Silicon on foil process focusing on all LTPS-TFT large-area imager applications. The circuit operates at 5V supply and draws 200μW power with a total readout interval of 50μs. With a dynamic range of 2V on the pixel photo-diode voltage, the integral non-linearity is measured to be 2% outperforming the current state-of-the-art for large-area imagers.