A 113.3-dB Dynamic Range 600 Frames/s SPAD X-Ray Detector With Seamless Global Shutter and Time-Encoded Extrapolation Counter

Abstract

This article presents a 400 x 200 single-photon avalanche diode (SPAD) X-ray detector that supports seamless global shutter operation. The pixel consists of an SPAD with two time-encoded extrapolation counters. For use under low X-ray dose conditions, the pixels and readout circuitry operate in a fully digital manner, avoiding the noise penalty of the readout. Under high X-ray dose conditions, the proposed counter operates in the extrapolation mode and thus achieves a high dynamic range (HDR), while reducing the counter's power consumption. This is achieved by capturing the time of the counter overflow (T-OF) and expanding the counter depth by the ratio between the integration time (T-INT) and T-OF. To reconstruct T-OF, a phi(GCLK) signal is distributed during T-INT and accumulated from T-OF to the end of T-INT by reusing the counter. This only requires distribution of the phi(GCLK) across the pixel array. Furthermore, seamless global shutter operation is realized through the use of two counters, which achieves a 100% temporal aperture for the X-ray photon. The proposed X-ray detector is implemented in a 1P5M 65-nm CMOS process. To be used for both medical diagnosis and industrial inspection, the pixel pitch is selected as 49.5 mu m. The 400 x 200 detector achieves a dynamic range (DR) of 113.3 dB, while consuming only 127.2 mW. Compared with previous X-ray detectors, the proposed detector exhibits a 24.9-dB improvement in DR.