Saturation in SPAD Indirect Time-of-Flight Imagers

Single-photon avalanche diodes (SPADs) are sensi-tive photodetector devices, often chosen to perform time-of-flight measurements for depth cameras because of their excellent timing properties. Indirect time-of-flight (iToF) sensors work by integrating the received signal in given time windows, then by deriving the phase from the measured photon counts. While SPAD iToF cameras give excellent results for short range measurements, they might suffer from signal deformation issues due to saturation when subjected to a high optical signal power. This saturation effect, its consequences on the signal waveform when received by the SPAD as well as on the time-of-flight distance measurements are explained and observed in this paper. A custom 0.18 µm CMOS IC containing a SPAD iToF pixel is operated under high optical power conditions to study this effect and its repercussions. Measurements confirm that the time-of-flight accuracy will be affected if two conditions are met: the photon detection rate is close to or greater than the inverse of the dead time, and close to or greater than the signal frequency.