A 72-bin in-pixel mixed-signal TDC for SPAD-based fluorescence lifetime measurements

Since the first observation of fluorescence phenomena, a wide variety of fluorescence measurement techniques have been developed for biomedical imaging. Therefore, low-cost and effective point-of-care devices are needed to characterize the fluorescence lifetime of desired samples. For this purpose, single photon avalanche diode (SPADs) CMOS imaging sensors have been used, which offer fast response and low fabrication cost. In this work, we present a CMOS SPAD pixel integrating a 10µm diameter SPAD and a 72-bin mixed-signal time-to-digital converter (TDC) with tunable bin widths from 100ps to 1ns in only 40µm x 40µm. The pixel architecture has been validated by performing fluorescence lifetime simulations of organic and inorganic dyes with various exponential decays below 100ns.