Dynamic range enhancement circuit and characterization of non-stationary gain effects in pulsed-reset charge sensitive amplifiers

In charge sensitive amplifiers (CSAs), a linear small signal charge-to-voltage conversion gain, is obtained connecting a capacitance Cf in feedback to a high gain voltage amplifier. However, for a sufficiently large input/output signal, the linearity assumption, valid for the neighborhood of the DC operating point (OP), is inevitably lost due to the inherent nonlinearity of internal active and passive elements of the circuit. In pulsed-reset CSA architectures [1], even small charge signals can stack on the feedback capacitance across the integration period, producing an output ramp which effectively moves the DC-OP, and slightly changes the small-signal gain over time up to the output saturation level, or until the reset phase occurs. Using different integrated CSAs, and mapping the small signal gain variations across the different DC-OPs through the integration ramp, we studied and modelled the effect on the spectroscopic resolution of the radiation detection system due to this non-stationary gain behaviour [2]. Experimental results show that, depending on the effective dynamic range selected during the measurement, even small variations in the conversion gain (<1%) for a limited portion of the output CSA voltage ramp can cause significant broadening and peak shift on the 5.9 keV 55Fe line. A circuital solution based on a voltage level shifting on the feedback discharge path to optimize the DC starting point of the CSA integration ramp is presented [3], allowing an increase of the effective linear dynamic range without affecting the spectroscopic resolution of the system.