The ORION Chipset for the X-Gamma Imaging Spectrometer Onboard of the THESEUS Space Mission

We present the design of a multichip Application Specific Integrated Circuit (ASIC), named ORION, for the front-end readout of the X-Gamma Imaging Spectrometer (XGIS) on-board the Transient High Energy Sky and Early Universe Surveyor (THESEUS) space mission. The XGIS instrument is composed by two cameras that operate as a wide field deep sky monitors with a broad energy range from 2 keV to 20 MeV, and it is based on a position sensitive double-detection mechanism for image reconstruction, in which a single pixel is constituted by a Thallium activated Cesium Iodide (CsI(Tl)) scintillator crystals and two Silicon Drift Detectors (SDDs) glued at both crystal ends, whose signal is collected, reconstructed and digitized by the presented ORION chipset. In each camera, the ORION chipset is organized in a constellation of 12 800 analog front-end chips (ORION-FE), closely connected to the SDD anodes, and 800 mixed signal multi-channel back-end chips (ORION-BE) for signal processing and digitalization, for a total 25 600 ORION-FE and 1600 ORION-BE in the complete instrument. The back-end chips have two parallelized X and Gamma signal processors, for low-energy and high-energy photons respectively, which allow a tailored optimization on the noise and energy range requirements for each type of event. The chipset has an input dynamic range of 32 fC that allows to process signals with a linearity error below ±1.2% on the Gamma processor, and below ±0.1% on the X processor. The nominal Equivalent Noise Charge (ENC) of the system at -20 °C for an estimated detector leakage current of 0.7 pA is 12.5 el. r.m.s at 1 µs peaking time for the X processor, and 32.9 el. r.m.s. at 3 µs peaking time for the Gamma processor. The simulated power consumption is of 1.55 mW per pixel.