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 104 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 104 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.