A Novel Active-Pixel Readout Circuit Based on Bias Current Cancellation for Dynamic X-Ray Imaging

This paper presents a novel readout circuit technique, which addresses the issue that the integration capacitors are easily saturated for current-mode active pixels. The proposed circuit utilizes a column-shared current source with threshold voltage compensation to cancel the static operating current of the active pixel. Consequently, the current flowing into the readout circuit consists solely of the changing current caused by X-ray exposure, avoiding that unwanted charges are injected into the ROIC, and significantly mitigating the demand for the integration capacitor. Furthermore, the DC offset of the current sources among columns can be eliminated through correlated double sampling, thereby improving uniformity. Compared to the conventional active-pixel readout circuit, the proposed one features a significant increase of the charge gain from 20.9 to 52.8 and a remarkable decrease of the nonlinearity from 1.83 % to 0.39 % with the integration capacitor of 8 pF and the ROIC voltage swing of 1.8 V.