Design of a Front-End Stage for Future European XFEL Detectors

The European X-Ray Free Electron Laser (XFEL) represents a significant advancement in photon science, enabling experiments featuring an excellent temporal and spatial resolution. XFEL sources provide extremely intense and ultra-short (in the order of femtoseconds) X-ray pulses. Within this framework, new X-Ray pixel detectors are required to satisfy the challenging requirements from XFEL experiments, including high frame rates, wide dynamic range, and low noise for single-photon sensitivity. In particular, achieving single-photon resolution while maintaining a large dynamic range of up to 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> photons presents substantial challenges in the design of the detector readout electronics. In this article, we present a novel front-end architecture based on a Charge Sensitive Amplifier (CSA) with predictive gain switching and a Flip-Capacitor filter. The CSA is designed for Silicon Pixel Detectors and aims to achieve both a single photon resolution and a high dynamic range up to 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> photons by employing an adaptive control of the gain. The novel predictive gain circuit dynamically sets the proper gain by measuring the time needed by the signal to exceed a fixed threshold. The gain is automatically adjusted by adding feedback capacitances in parallel. When the CSA is in its highest gain configuration, a filter is connected to reduce the electronic noise and achieve the single-photon resolution below 1 keV. The filter is based on the Flip-Capacitor Filter concept and implements a fast trapezoidal weighting function. To couple the CSA with the filter, a voltage-to-current converter has been designed. A first prototype of this front-end has been designed in 65 nm CMOS technology, and the design and simulations are presented here.