Histogram-based mixed-signal time-to-digital-converter array for direct time-of-flight depth sensors

This Letter introduces a histogram-based time-to-digital-converter (TDC) array for direct time-of-flight depth sensors. The 12-bit TDC array measures the time of flight (TOF) of a light pulse that is detected using an external single-photon avalanche diode (SPAD) array. As SPADs are noisy owing to dark electrons and scattered photons, statistical measurements based on histograms are essential, but require a large memory. In this work, the authors propose a mixed-signal TDC with an integrated histogram generation unit (HGU) that reduces memory requirement significantly as well as filters out invalid TOF readings. In addition, for application of the high-resolution SPAD array, an area-efficient TDC array with high uniformity was implemented by the proposed phase-dependent latching and temporal double sampling schemes. The prototype chip was fabricated using a 180 nm CMOS process, including 8-channel TDCs. The measurements show an integral non-linearity (INL)/a differential nonlinearity (DNL) of 0.76/0.49 least significant bit (LSB) and high uniformity under 0.19 LSB. The HGU was designed off-chip for prior verification, and was post-simulated with the measured TOF from the fabricated chip. Using the histogram-based TDC, the authors could detect an object located at a distance of 3 m accurately while reducing memory requirement by more than 128 times.