An 80 x 60 Flash LiDAR Sensor With In-Pixel Delta-Intensity Quaternary Search Histogramming TDC

Abstract

This article presents a flash light detection and ranging (LiDAR) sensor featuring an in-pixel histogramming time-to-digital converter (hTDC) based on a delta-intensity quaternary search (DIQS) technique. The proposed 12-b DIQS hTDC is a two-step converter consisting of a 6-h coarse hTDC and a 7-b fine hTDC with 1-b redundancy. The DIQS hTDC synthesizes depth maps with three subframes from the coarse mode and a single subframe from the fine mode, achieving 100-ps resolution without a clock frequency of a few gigahertz. The DIQS repeats dividing the time range of a current step into four periods and finding the location where a target object is placed by comparing the number of events in each period, which is similar to the binary search method but doubles its operating speed. Two time-of-flight (ToF) bits are consecutively determined in every coarse step, and seven ToF bits are estimated by the indirect ToF technique with photon counts. An up-down counter is employed to reduce the memory size by half and enable the delta-intensity technique that can extend the dynamic range by suppressing the uniform background light. The prototype LiDAR with an 80 x 60 pixel array was fabricated in a 110-nm CMOS image sensor (CIS) process and fully characterized. The maximum detectable range is measured to 45 m with a success rate of 100% at night and 60% under 70-klux background light. The depth accuracy and precision are 2.5 and 1.5 cm from 3 to 4.5 m indoor, respectively, and the precision is maintained to 1.8 cm for the target located at a 1.5-m distance under 60-klux background light. Inherent time-gating and differential signaling of the DIQS hTDC effectively suppress common-mode noise, accomplishing real-time acquisition of depth images with 30 frames/s in a 9-m range at 30-klux background light.