TOT_AL: An ASIC for TOF and DOI measurement

We present the architecture of a new ASIC intended to read-out matrices of Silicon Photomultipliers coupled to continuous slabs of scintillating crystals for medical imaging applications. Time Of Flight (TOF) measurements, xy position and Depth Of Interaction (DOI) evaluation for the gamma photon require setting the detection threshold down to the level of one single photon and accurate estimation of the energy of the event. We propose a large bandwidth, current mode analog front-end able to discriminate valid events from the dark pulses generated by the SiPM on the basis of the charge contained in the detector signal. The LVDS output of the channel provides a rectangular pulse with a very fast leading edge, which marks the arrival of the event, and duration that is linearly related to the total charge generated by the detector, thus implementing a sort of Time over Threshold (ToT) technique. Thus, an external TDC can be used to extract both time and energy information. A 5 channel prototype ASIC, called TOT_AL, has been designed and the timing accuracy achieved in post-layout simulations is about 235ps FWHM, with single photo-electron resolution in the charge measurements. We describe the structure and the operating mode of the analog channel and the hardware solutions that have been exploited in order to reach the required specifications in terms of timing accuracy and linearity of the energy measurements.