Mutually Coupled Ring Oscillators for Large Array Time-of-Flight Imagers

Time-of-flight (TOF) image sensors require high levels of accuracy and precision on time quantization, typically through a Time-to-Digital Converters (TDCs). Achieving such performance, while keeping power consumption low, is not trivial, even more for large arrays. Although offsets (from asymmetry, overall speed variation, IR-drop induced variations, etc.) can be calibrated and corrected, varying offsets are harder to compensate for. In this paper, we present a simple and robust method for synchronization that drastically reduces process-voltagetemperature (PVT) variations, while providing noise filtering to reduce accumulated jitter. An 8 x 8 array of ring oscillators (RO), serving as many TDCs, is implemented. About 18 dB better phase noise and 14 dB better jitter is obtained, under 200 μW/GHz power efficiency and better than 100 ps resolution. Index Terms — ring oscillators, coupled-oscillators, brusselator, time-to-digital converter, TDC, LIDAR, time-offlight, TOF, noise reduction.