Capacitor based multiplexing circuit for silicon photomultiplier array readout

Several different multiplexing readout methods have been investigated for reading out silicon photomultiplier (SiPM) arrays. However, it is still challenging by using these methods to maintain signal integrity for overall good signal and imaging performance while reducing the number of readout and processing channels. One common issue to resistor based multiplexing method is the position-dependent timing shift among different channels, which can in principle be calibrated and corrected but add complexity to the detector calibration and operation process, and can be very difficult to apply for a practical PET system for routine imaging applications. To solve such and other problems, we explored a capacitor-based multiplexing method for our PET detector to read SiPM with a common cathode which has not been addressed previously. To achieve good detector performance, we required output signal without undershot/overshot suited for excellent charge integration, and without timing shift among different channels. The design applies a capacitor network to divide the charge of signals from a SiPM into two branches, with the division of charge based on the position of the SiPM in the network. Only one capacitor value is needed. The number of readout channels can be reduced from N×N to 2N. Evaluation circuit was tested with pulsed signals and a practical PET detector which consisted of an 8×8 SiPM array and LYSO scintillator array. The results showed that signal rise and fall times from different channels were the similar, no output signal undershot, and no timing shift among different channels. The resistive and capacitive multiplexing methods were compared for their noise level, energy resolution, rise time, and timing resolution as function of channel numbers. Capacitive multiplexing method shows better noise and timing performance with much better timing and energy consistent from all detector area. A PCB circuit board with capacitor multiplexing has been developed for PET detector applications.