Design and characterization of a 32-channel front-end ASIC for Silicon Photo-Multiplier detectors

In many detector systems in which it is required to reveal the presence of high energy y photons, Silicon PhotoMultipliers (SiPM) have gained a dominant role over classical Photo-Multiplier Tubes (PMT) thanks to their much lower bias voltage (30 - 60V vs > 700V of PMT's), compact size and immunity to magnetic fields. These characteristics make SiPM's suitable for modem medical imaging systems such as Positron Emission Tomography coupled to Magnetic Resonance (PET/MRI) ones. However, the range of application of these devices extends to many other fields such as, for example, security systems (luggage scanners and dirty bomb detectors) high energy physics (e.g. calorimeters), astrophysics and space science. To fully exploit the excellent performance of SiPM's in terms of quantum efficiency, high gain, low bias voltage, and timing accuracy, suitable integrated front-end solutions and smart read-out architectures have to be devised. Here we present a current mode CMOS integrated frontend solution which has been implemented in a 32-channel ASIC. The chip has been designed and manufactured in a standard 0.351lm CMOS technology and has been extensively characterized using an external injection capacitance, proving that the architecture of the analog channel allows to achieve very good performance in terms of dynamic range (around 70pC), bandwidth and timing accuracy (O"=:llSps). Moreover, the ASIC has been used in a self-triggered mode to read-out a SiPM from Hamamatsu, coupled to a small L YSO scintillator, and the resulting spectra obtained by exposing the detector to different radiation sources confirm the effectiveness of our design approach. Remarkable dynamic range and good linearity have been experimentally achieved in all the measurements performed.