A Pixel Analog-Counting Single-Photon Imager With Time-Gating and Self-Referenced Column-Parallel A/D Conversion for Fluorescence Lifetime Imaging

Abstract

A single-photon, time-gated, $160 \times 120 $ pixel imager is presented for its application in fluorescence lifetime imaging microscopy. Exploiting single-photon avalanche diodes and an extremely compact pixel circuitry—only seven MOSFETs and one MOSCAP—the imager is capable of gathering information about photon position, number, and time distribution, enabling cost-effective devices for scientific imaging applications. This is achieved thanks to the photon counting and time-gating capabilities implemented in the analog domain, which in turn enable a $15\;{\upmu}{\text{m}}$ pixel with a 21% fill-factor. A reconfigurable column circuitry supports both the analog conventional readout and a self-referenced analog-to-digital conversion, able to cancel out the pixel-to-pixel nonuniformities, and speeding up the framerate to 486 fps. The imager, featuring also a delay locked loop to stabilize the internal waveform generation for reliable timing performance, has been implemented in a standard high-voltage $0.35\;{\upmu}{\text{m}}$ CMOS technology. Measurements in a fluorescent lifetime setup have been performed, comparing the results with single-point acquisitions made with commercial time-correlated equipment.