256 x TDC Array With Cyclic Interpolators Based on Calibration-Free 2x Time Amplifier

This paper presents the design of a CMOS time-to-digital converter (TDC) used in a direct time-of-flight laser radar line receiver consisting of an array of single-photon avalanche diodes (SPADs) and 256 TDCs. The TDCs use the Nutt interpolation method, where the start/stop fine interpolators are based on a cyclic converter architecture. The cyclic converters use a delay-line-based time interval amplifier to double the quantization residue after each quantization cycle. The time amplifier, and the entire TDC array, does not need any biasing nor does the TDC results need any post-processing to provide sub-100-ps precision and uniformity across the TDC array, therefore making the design very robust and appealing for large designs where hundreds of TDCs are needed on a single chip. Fabricated in a 0.35 μm CMOS technology, a single TDC channel occupies an area of 0.03 mm² and consumes a power of 1.17 mW @ 100 kHz measurement rate. The TDC has a measurement range of 640 ns, and with a resolution of 15 bits, the LSB is 20 ps. Linearity is ensured by the Nutt interpolation method while the rms single-shot precision of the TDC channel is 72 ps. Thus, when LIDAR applications are considered, the TDC is capable of providing a range of 96 m with a precision of 1 cm without any post-processing or calibration.