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Monolithic time-to-amplitude converter for photon timing applications

In the last years Time-Correlated Single-Photon Counting (TCSPC) has increasingly been used in many different scientific applications (e.g.: single molecule spectroscopy, fluorescence lifetime imaging, diffuse optical tomography). Many of these applications are calling for new requests on the development of instrumentation that operates at higher and higher conversion rates and that is able to resolve optical signals not only in the time domain, but also in wavelength, polarization and position. To exploit the potential of parallel analysis over multiple acquisition channels, a new generation of TCSPC devices is needed that is characterized by low size and costs. The core block of TCSPC instrumentation is the time-interval measurement section, which can be implemented with a Time-to-Amplitude Converter (TAC); the converter can be integrated on a single chip in order to reduce the overall size and cost of the system. This paper presents a monolithic TAC that has been designed to achieve the high resolution, good differential linearity and fast counting rate required in modern applications. The TAC here described is built on a commercial 0.35 μm CMOS technology, and is characterized by resolution better than 60 ps, differential nonlinearity limited to 0.5% rms and short dead-time of 80 ns. The low area occupation (1.4x1.8 mm) and minimal need for external components allow the realization of very compact instruments with multiple acquisition channels operating simultaneously at very high count rates.