GAMMA: a 16-Channel Spectroscopic ASIC for SiPMs Readout with 84dB Dynamic Range

Gain amplitude modulation multichannel ASIC (GAMMA) is a 16-channel application specified integrated circuit (ASIC), designed in 0.35- $\mu \text{m}$ technology to read the charge provided by silicon photomultipliers (SiPMs) on a wide amplitude range. This chip was designed to satisfy the demanding requirements of the INFN the GAMMA project, where a 3' $\times3$ ' large lanthanum bromide crystal is to be read by SiPMs, aiming at state-of-the-art energy resolution. Because the energy range of the application is 100 keV-20 MeV, signals in a wide dynamic range (DR) have to be processed by the ASIC. This wide DR requirement raised the necessity of an adaptive gain change solution in order to guarantee a high signal-to-noise ratio together with a large full-scale range. While the ASIC input stage features a programmable static gain, the analog filtering channel exploits a self-triggered gated integrator stage with an automatically adjusted gain for each detected input current pulse; one of the following values is selected: 10, 2, or 0.83 [mV/pC]. The gated integrator gain is predictively adjusted before the circuit reaches saturation due to a time-gated threshold mechanism and provides negligible charge loss when the gain is switched (< 20 fC). A preintegration feature is also implemented in order to get rid of nonlinearity due to delays in the integration phase that could take place in a multi-ASIC system. The ASIC uses 5- and 3.3-V bias voltages, the overall power consumption being 280 mW. The ASIC was tested with a $12\times12$ SiPMs matrix coupled with a 3 inch LaBr 3, providing good spectroscopic performances (2.6% at the 137Cs photopeak emission energy of 662 keV), and was also tested irradiating the SiPMs with fast, bright NUV light pulses, in order to explore the ASIC wide range coverage while simulating the scintillation light emission of a lanthanum bromide crystal.