Depth-range extension with folding technique for SPAD-based TOF LIDAR systems

Direct Time-of-Flight (D-ToF) sensors can be implemented combining Single-Photon Avalanche Diodes (SPAD) with Time-to-Digital Converters (TDCs). Such an architecture exhibits an intrinsic distance range limitation due to the TDC range, which is fixed by the TDC resolution and number of bits. Up to now it was not possible to extend this range without redesigning the TDC. In this work we propose a method to overcome this limitation thanks to a folding measuring technique. In our implementation the TDC is based on a ring-oscillator; the TDC starts when a photon is detected and stops when an external Stop signal is provided. As the number of bits is limited, the TDC restarts from 0 after reaching the final value (folding). In this paper, we show that it is possible to arbitrary extend the maximum detectable distance through repeated measurements by modifying the timing of the “stop” signal in multiples of the TDC´s maximum range.