Design of a fast-shaping amplifier for PET/CT APD detectors with depth-of-interaction

An integrated 0.35-μm CMOS fast-shaping amplifier has been designed for coincidence detection and zero-cross time pulse-shape discrimination (PSD) in positron emission tomography (PET) and for high-rate event counting in computerized tomography (CT) with avalanche photodiode (APD)-based detectors. Analytical simulations of CR-RCⁿ filters of various order and shaping time constant were carried out to optimize the timing performance, keeping in mind the stringent channel density requirements of the detector front-end electronics. The filter was implemented with the biquadratic bandpass architecture using a folded cascode transconductance amplifier. The electronic timing resolution of the coincidence circuit was 92 ps in the ideal case without an APD (C_in = 0 pF), and 243 ps with an APD (C_in = 30 pF). By comparison, the same system with the CMOS shaper replaced by an Ortec 579 Fast Filter Amplifier set to a shaping time of 10 ns yielded an electronic time resolution of 79 ps in the ideal case without capacitance and 236 ps with the APD. The 511-keV coincidence time resolution for a crystal scintillator-based lutetium oxyorthosilicate (LSO)-APD detector was measured to be 1.49 ns.