Development of a 64-channel front-end ASIC for accurate and robust crystal identification of high-resolution PET detectors

We have developed a 64-channel analog front-end ASIC (application specific integrated circuit) for reading out signals from PS-PMT (position sensitive photomultiplier tube) based high-resolution PET (positron emission tomography) detectors. The ASIC has been designed for accurate and robust crystal identification with Anger logic even under high-count rate conditions. To achieve the feature, a clamp amplifier was built into each readout channel. The whole circuit consists of the front-end containing 64-readout channels, an Anger-type position calculator, and an energy summing amplifier. The readout channel is comprised of current-mode VGA (variable gain amplifier), I/V amplifier, clamp amplifier, and offset controller. In addition, each channel can perform digitally programmable adjustment of gain and offset. The ASIC has been fabricated in the New Japan Radio 0.8-um 2P3M FD-SOI BiCMOS technology and the chip size is 10-mm square. To reduce the mounting area of the ASIC, 196-pin 13-mm square FC-PFBGA (flip chip plastic fine pitch ball grid array) was employed as package of the ASIC. Using this ASIC, we have developed a front-end circuit board for a high-resolution PET scanner and evaluated basic characteristics with a four-layer DOI (depth of interaction) detector that consists of 4,096 LGSO crystal elements and a 64-channnel PS-PMT. As a result, by using the clamp amplifier, each crystal element on the two-dimensional position histogram was clearly identified even at the count rate of 245 kcps.