High precision time-to-amplitude converter for diffuse optical tomography applications

Molecular imaging aims to better understand biochemical processes at the molecular level in living organisms. Among emerging imaging modalities, diffuse optical tomography (DOT) coupled with the use of fluorescent bio-markers proposes solutions to visualize such processes. The Universite de Sherbrooke is developing a 128-channel DOT scanner dedicated to small animal imaging. The scanner is based on ultra-fast time-resolved optical measurements using time-correlated single photon counting (TCSPC). This paper reports a preliminary design of a high resolution 4- channel time to amplitude converter (TAC) used for time of flight optical measurements, in CMOS 0.18 mum technology. The TAC achieves a 5 ps resolution in a 13 ns timing window, and has a 150 ns dead time. It includes all auxiliary electronics: D-flip-flop, current source, integrator, switches and fully balanced buffer able to drive an off-the-shelf free running 50 Msps analog to digital converter (ADC).