Pole-Zero Cancellation Circuit With Pulse Pile-Up Tracking System for Low Noise Charge-Sensitive Amplifiers

Modern X-ray imaging systems require low noise high-count-rate multichannel front-end electronics. Such systems widely use charge-sensitive amplifiers (CSA) with pole-zero cancellation (PZC) circuits for sensor readout. CSAs in multichannel ASICs often include continuous reset systems based on MOS transistors discharging the integrating capacitance. Noise requirements for such front-end electronics put the effective CSA feedback resistance in the range of hundreds of MOmega. On the other hand, the high input pulse rate generates a dc voltage shift at the CSA output that modifies the CSA feedback resistance and degrades the PZC circuit performance. In the paper we analyze these problems and propose a novel circuit solution to bias transistors in the CSA feedback and in the PZC circuit. This bias circuit tracks the above mentioned voltage shift at the CSA output and ensures proper cancellation of the CSA feedback pole even in the case of high rates of input pulses. We present measurements of a test structure for the continuous CSA feedback discharge and implementation of this circuit in a fast multichannel ASIC manufactured in the 0.35 mum CMOS technology. The circuit performance for the high input pulse rate is described in detail.