An automatic pole-zero adjustment circuit is provided for an ionizing radiation spectroscopy system which comprises a pole-zero network circuit, coupled to the system and having an electrical chracteristic which varies as a function of an input control signal, for permitting pole-zero adjustment of the system; a sampling circuit coupled to the system, for sampling a segment of the trailing edge of random pulses passing through the system; and a control signal circuit coupled to the sampling circuit, for generating the input control signal as a function of the sampling to permit cancellation by the pole-zero network circuit of undershoot in the system. The pole-zero network circuit may include a real pole in the signal path of the network circuit set coincident with the real pole of a high pass filter in the system itself. The control signal provided to the pole-zero network circuit is digitized.