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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$400 \times 200$ </tex-math></inline-formula> 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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm{ OF}}$ </tex-math></inline-formula> ) and expanding the counter depth by the ratio between the integration time ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm{ INT}}$ </tex-math></inline-formula> ) and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm{ OF}}$ </tex-math></inline-formula> . To reconstruct <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm{ OF}}$ </tex-math></inline-formula> , a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\phi _{\mathrm{ GCLK}}$ </tex-math></inline-formula> signal is distributed during <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm{ INT}}$ </tex-math></inline-formula> and accumulated from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm{ OF}}$ </tex-math></inline-formula> to the end of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm{ INT}}$ </tex-math></inline-formula> by reusing the counter. This only requires distribution of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\phi _{\mathrm{ GCLK}}$ </tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text {m}$ </tex-math></inline-formula> . The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$400 \times 200$ </tex-math></inline-formula> 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.