A 20ch TDC/ADC hybrid SoC for 240×96-pixel 10%-reflection <0.125%-precision 200m-range imaging LiDAR with smart accumulation technique

Long-range and high-pixel-resolution LiDAR systems, using Time-of-Flight (ToF) information of the reflected photon from the target, are essential upon launching safe and reliable self-driving programs of Level 4 and above. 200m long-range distance measurement (DM) is required to sense proceeding vehicles and obstacles as fast as possible in a highway situation. To realize safe and reliable self-driving in city areas, LiDAR systems uniting wide angle-of-view and high pixel resolution are required to fully perceive surrounding events. Moreover, these performances must be achieved under strong background light (e.g., sunlight), which is the most significant noise source for LiDAR systems. To accomplish a 100m-range DM, an accumulation of the DM results through several pixels is utilized to improve the S/N ratio with 70klux background light [1]. Here, S is the number of photons reflected from the target and N as the number of background light photons. However, if the range is extended to 200m under similar condition of the laser power and frame rate (FPS), 16x more pixel accumulation is required. Such pixel accumulation leads to blurring the range image, and hence, a serious oversight in the surrounding events, such as a flying-out pedestrian, may occur, not suiting self-driving applications. Furthermore, the Time-to-Digital Converter (tDC) based ToF measurement is activated only when 2 or more photons are detected simultaneously [1], and thus, is not suitable for the 200m long-range DM where few photons are reflected from the target. On the other hand, ToF measurements using ADCs, which can continuously quantize the silicon photomultiplier (SiPM) output and can sense single-photon events, suits long-range measuring purposes well [2]. However, a number of accumulations should still be required to accomplish 200m-range DM, and hence, low resolution is inevitable. In addition, the SoC cost is critical. To enhance the short-range DM resolution by using ADCs, the required sampling rate is over 10GS/s; upon realizing a 20ch AFE, such an ADC array alone may occupy an area of over 10mm² and consume huge power [3].