An 8-windows Continuous Wave Indirect Time-of-Flight Method for High Frequency SPAD-based 3D Imagers in 0.18 μm CMOS

Time-of-flight (ToF) Single-Photon Avalanche Diode (SPAD) arrays are an increasingly popular option for 3D imaging. However, the traditional direct (dToF) and indirect (iToF) SPAD ranging pixels still struggle to get accuracies and precisons below the millimeter for short ranges. We propose to adapt the continuous-wave indirect time-of-flight (cw-iToF) method to use 8 integrating windows instead of the usual 4. This 8-windows cw-iToF method greatly reduces the non-linearity error when using a square optical signal, which consequently enables its use at much higher frequencies than if a sine wave was required, while also showing great robustness to an imperfect square wave, unlike pulse indirect time-of-flight (p-iToF). We provide an analysis of the effects of this new method on the accuracy and precision. Then, we present a single-pixel proof of concept implementation of an 8-taps cw-iToF SPAD pixel containing 4 up/down counters in a 0.18 μm CMOS process. Laser diodes illuminate the target with a 200 MHz square wave. The depth measurement’s worst non-linearity error over a 75 cm range is 1.2 mm, and stays below one millimeter for more than 90% of the measured range. Furthermore, the precision for 100 ms-long frames is as low as 0.35 mm.