Ultra Compact and Low-power TDC and TAC Architectures for Highly-Parallel Implementation in Time-Resolved Image Sensors

We report on the design and characterization of three different architectures, namely two Time-to- Digital Converters (TDCs) and a Time-to-Amplitude Converter (TAC) with embedded analog-to-digital conversion, implemented in a 130-nm CMOS imaging technology. The proposed circuit solutions are conceived for implementation at pixel-level, in image sensors exploiting Single-Photon Avalanche Diodes as photodetectors. The fabricated 32x32 TDCs/TACs arrays have a pitch of 50μm in both directions while the average power consumption is between 28mW and 300mW depending on the architectural choice. The TAC achieves a time resolution of 160ps on a 20-ns time range with a differential and integral non-linearity (DNL, INL) of 0.7LSB and 1.9LSB, respectively. The two TDCs have a 10-bit resolution with a minimum time resolution between 50ps and 119ps and a worst-case accuracy of ±0.5 LSB DNL and 2.4 LSB INL. An overview of the performance is given together with the analysis of the pros and cons for each architecture.