Testing and Calibration Methods for Hybrid PMT/SiPM Gamma-Ray Camera Read-Out Electronics

AdaptiSPECT-C is a human brain single-photon emission computed tomography (SPECT) system with 25 stationary scintillation gamma-ray cameras. Each camera has a dedicated read-out electronics system. The back-end electronics consist of a main board which includes, among other features, slots for a system-on-a-module (SoM) and slots for three A/D modules. Each A/D module consists of 28 1-bit sigma-delta modulators (SDMs). Each camera has 25 photomultiplier tube (PMT) channels and 56 silicon photomultiplier (SiPM) channels. Two A/D modules are utilized for the SiPM channels, and one for the PMT channels. We have designed an A/D module tester board for debugging, testing, and calibrating the A/D channels. The tester board takes an input signal from a waveform generator and creates 28 identical copies of the signal. The 28 generated signals are sent to the input of the SDM channels. For rapid testing, the SDM threshold for all channels is controlled by a potentiometer and the DC offset by another potentiometer. The output modulation is sent to bicolor LEDs, where the combined color of the LED pair depends on the SDM rate, which allows us to visually detect modulation variations. The output of the SDM comparator is fed back to the SDM input directly, so that no digital logic is needed for testing. The tester board includes a connector that outputs the 28 generated identical signals to be used as signal input for the main back-end board. This testing method offers a convenient and feasible process to rapidly test, debug, and calibrate the more than 2000 A/D channels in the AdaptiSPECT-C system.