Time-to-digital converters for PET: An examination of metrology aspects

Time-to-digital converters (TDCs) occur in many realms of high-energy physics, such as time-of-flight positron emission tomography (TOF PET)^[1]. Density tests^[2] are often used to characterize the non-uniformity of TDCs. Such tests require a uniform time interval generator, often realized by a probabilistic exponential source, such as a single-photon avalanche diode (SPAD), and a known time reference. However, if the exponential source´s event rate is too large, static distortions will occur in the measured INL. Additionally, the correlated shot noise created by the probabilistic source will introduce some uncertainty into measurements of the differential nonlinearity (DNL) and integral non-linearity (INL). This paper will discuss these two unwanted effects, and present measurements showing that the analysis is correct.