ANGUS: A multichannel CMOS circuit for large capacitance silicon photomultiplier detectors for SPECT applications

Clinical SPECT/MRI applications based on indirect conversion require the use of large area solid-state detectors. Silicon Photomultiplier (SiPM) is a fast-improving technology suitable for this application, due to its compatibility with the MRI environment and to its high multiplication gain. Considering the large number of detectors of a SPECT ring, a multichannel ASIC is needed to have a compact readout. SiPMs have a large specific output capacitance (approx. 200 pF·mm^-2), which can result in a load of several nanofarad for each pixel at the input of the readout circuit. We developed ANGUS, a 36-channel 0.35 μm CMOS technology ASIC designed to cope with input capacitances larger than 10 nF, with an overall power dissipation of 400 mW. A current mode input is used, allowing an input voltage tuning for SiPM overvoltage regulation in a 1 V range. A programmable RC filter is used to extract the signal amplitude information. The implemented time constants make the chip suitable for use with various type of scintillator crystal (e.g. CsI:Tl, GaGG:Ce, LYSO:Ce, NaI:Tl). Room temperature energy resolution measurements performed with ANGUS employing SiPM from FBK and Hamamatsu coupled to CsI:Tl crystals with a ⁵⁷Co source are reported in the following. 12% FWHM resolution was measured at the 122 keV line of this source using FBK detectors.