Designing a new data acquisition circuit for SiPM-based detection systems—MexSiC

Silicon Photomultipliers (SiPM) are currently an excellent option to replace the traditional photomultiplier tubes (PMT) due to the fact that they require much lower operational voltages than the PMTs with photon detection efficiencies (PDE) higher than 40% at peak wavelengths (in the blue-green visible part of the spectra), do not require the electromechanical complexity related with PMT based solutions, and are insensitive to magnetic fields. However, one disadvantage of SiPMs, if used in applications where huge detection areas are required, is their respectively small photoactive area (of few square millimeters). Modular approaches based on arrays of SiPMs are normally used in such cases. In order to use SiPM arrays, the modular data acquisition (DAQ) systems must be capable of reading multiple channels in parallel, synchronizing the times of arrival of all SiPM output pulses, processing these pulses, discriminating between events, and generating resulting digital output signals that can be stored on an external memory unit. An application specific integrated circuit (MexSIC) to be fabricated in the 180 nm CMOS technology has been designed as a core unit of such a DAQ system. It contains a transimpedance amplifier (TIA) as its input stage, a triggering logic unit (TLU) used to discriminate the input signals, a phase locked loop (PLL) used to generate a clock reference for the time-to-digital converter (TDC) and an additional charge-to-digital converter (QDC), used for discrimination of detected events based on the amount of charge generated within each individual SiPM on the one side, and the duration of each event on the other, as well as a Delta-Sigma ($\Delta$-$\Sigma$) analog-to-digital converter (ADC). An additional field-programmable gate array (FPGA), the Xilinx Kintex 7, is used for signal processing and system controlling. We present the concept of the proposed DAQ system.