Recent Developments and Design Challenges of High-Performance Ring Oscillator CMOS Time-to-Digital Converters

Time-to-digital converters (TDCs) are increasingly used as building blocks in biomedical imaging, digital communication, and measurement instrumentation systems. When fabricated in deep-submicrometer (DSM) CMOS technology, TDCs have outstanding time stamping capability on the order of picoseconds. Typically, the timing resolution of a TDC directly determines the minimum resolvable spatial resolution in time-of-flight (ToF) measurements. It also limits the signal-to-noise ratio in ToF positron emission tomography and the in-band noise in an all-digital phase-locked-loop. In TDCs, good linearity and precision result in high measurement accuracy, while the detectable range is limited by its dynamic range. In addition, size and power consumption are of significant importance in large-scale array implementations such as image sensors. Here, we discuss the most recent developments in CMOS TDCs, with an emphasis on ring-oscillator-based TDC and its variants, due to their suitability for array designs with less area overhead. In addition, key performance metrics, and accurate cost-effective characterization methods will be discussed. Finally, future perspectives of CMOS TDCs will be highlighted.