A 32-Channel Time-to-Digital Converter with 20-ps Resolution for ToF Applications

Single-photon avalanche diodes (SPAD) enable sensitive detection of single photons under ultra-weak light conditions. Combined with direct time-of-flight (ToF) measurements, SPAD sensors are widely used in 3D imaging systems. Time-to-Digital Converter (TDC) can accurately measure time intervals in picoseconds, which is widely used for direct TOF measurements. This paper proposes a 32-channel TDC of two-level architecture containing a delay locked loop (DLL) and a vernier delay line (VDL) loop to increase timing accuracy. Designed in a 180 nm CMOS process, the TDC occupies an area of 4.1 mm $\times$ 3.6 mm and achieves 20 ps resolution at 2 Msps measurement rate.