AN-DPP-001: Digital Pulse Processors Theory of Operation

Amptek Inc. Page 1 of 6 Digital Pulse Processors, Theory of Operation R. Redus, Chief Scientist Amptek Inc., November, 2009 Digital Pulse Processors (DPP) are now widely used in high performance nuclear instrumentation and are at the heart of most of the systems sold by Amptek, Inc. A DPP performs the same functions as an analog shaping amplifier but has intrinsic performance advantages which have led to its widespread adoption where both the lowest noise and the highest count rates are needed. Although the function of the DPP is the same as an analog shaper, the implementation is different and some of the concepts and terminology are different. The purpose of this application note is to compare analog and digital shapers, to aid users in understanding digital processors, and to explain their advantages and disadvantages. Simplified Schematics Figure 1 and Figure 2 show simplified schematics of analog and digital shaping amplifiers, respectively. Both have the same detector and charge sensitive preamplifier circuits. In both cases, the preamplifier produces an output which consists of small steps, millivolts in height. In both cases, the preamp pulses are differentiated so that the step voltage can be measured. An integrator (a.k.a. low pass filter) improves the signal-to-noise ratio. In both cases, the output pulses are digitized and a histogram of the pulse heights is stored in memory. These key elements are the same in both systems.