A charge sensitive amplifier (CSA) is widely used as the preamplifier of an X-ray or gamma-ray detector. It is based on a low-noise high frequency field-effect transistor and a current feedback operational amplifier allowing the readout of fast charge pulses.
The pile-up of output pulses in a CSA is attributable to its long discharge time constant. A CR differentiator which shortens pulse width is used to remove pile-up. Due to the exponential decay of a CSA feedback circuit in combination with a differentiator circuit, undershoot occurs at the output of a shaping amplifier with the same time constant as the CSA. PZC circuit is inserted between the CSA and the shaping amplifier to eliminate undershoot. But this makes it difficult to decide the real differential time constant of the CR differentiator after elimination of undershoot and to process the signals like peak detection, time analysis, etc. An analog PZC circuit always has some uncertainties that result in pulse tail pile-up. This drawback can be overcome with a digital pole-zero, zero-pole compensation unit.
A digital PZC method also has some uncertainties. Although digital real time processing of pulses could, in principle, eliminate almost all pulse pile-up distortion in spectroscopy, it is not yet widespread.
Pae Won Sik, a researcher at the General Assay Office, has performed an analysis to search for the cause of undershoot after differentiation of input signals and designed a new PZC circuit with unchanged differential time constant by using a pot resistor. Then, he has conducted some simulations and experiments to verify its successful performance.
The results show that the proposed circuit effectively eliminates undershoot and significantly reduces pulse width, which leads to the improvement of pile-up cancelling, peak detection and time analysis.
© 2021 KumChaek University of Technology