TACImager - a high frame rate 320 x 256 SPAD time to amplitude converter array with adjustable time zoom

Single-photon avalanche diodes (SPADs) in the form of high-resolution imaging pixel arrays are used in 3D cameras, motion-tracking, biomedical and time-correlated single photon counting (TCSPC) applications. Rapid spatial and temporal zoom onto objects of interest is an attractive feature. We present here novel high-speed time-zoom functionality achieved with the digital readout mode of the TACImager, a 256 x 256 TCSPC image sensor array based on sample and hold Time to Amplitude Converter (TAC) pixels. A column-parallel flash Analogue to Digital Converter (ADC) is implemented in the TACImager to support fast digital readou t, allowing per-pixel, 3-bin TCSPC histogramming at frame rates of 4 kfps. New results related to this high-speed mode of operation are presented. The TACImager utilises a global ramp voltage as a timing reference, allowing time-zoom to be achieved through dynamic adjustment of comparator voltages, ramp offset voltages and ramp waveforms. We demonstrate the influence of fixed pattern noise in the pixels and column parallel ADCs on the results.