A New Statistics-Based Online Baseline Restorer(SOBLR) for a High Count-Rate Fully DigitalSystem

摘要

This study is to develop a novel, accurate, real-time digital baseline restorer using online statistical processingfor a high count-rate digital system such as positron emissiontomography (PET). In high count-rate nuclear instrumentationapplications, analog signals are DC-coupled for betterperformance. However, the detectors, pre-amplifiers and otherfront-end electronics would cause a signal baseline drift in aDC-coupling system, which will downgrade the performance ofenergy resolution and position accuracy. Event pileups normallyexist in a high-count rate system and the baseline drift willcreate errors in the event pileup-correction. Hence, a baselinerestorer (BLR) is required in a high count-rate system toremove the DC drift ahead of the pileup correction. Manymethods have been reported for BLR from classic analogmethods to digital filter solutions. However a single channelBLR with analog method can only work under 500KCPS count-rate, and normally a CMOS front-end ASIC is required for theapplication involved hundreds BLR such as a PET camera. Wehave developed a simple statistics-based online baseline restorer(SOBLR) for a high count-rate fully digital system, andhundreds SOBLRs can be implemented in one field-programmable gate array (FPGA). In this method, we acquireadditional samples, excluding the real gamma pulses, from theexisting free-running ADC in the digital system, and perform anonline statistical processing to generate a baseline value. Thisbaseline value will be subtracted from the digitized waveform toretrieve its original pulse with zero-baseline drift. This methodcan self-track the baseline without a micro-controller involved.The circuit consists of two digital counter/timers, onecomparator, one register and one subtraction unit. Simulationshows a single channel works at 30MCPS count-rate with pileupcondition. 336 baseline restorer circuits have been implementedin FPGA for our new fully digital PET system.