A simple technique for signal compression in high dynamic range, high speed X-ray pixel detectors

The availability of high-intensity and high repetition rate X-ray sources, like XFEL facilities, impose severe constraints for the detectors to be developed, in terms of high speed and high dynamic range. For the European XFEL, different detector developments are on the way to readout X-ray flashes with a repetition rate of 4.5 MHz with a dynamic range up to 104. In this framework, different compression techniques, at the sensor level or at the electronics readout level, have been adopted to allow the detection system to cope with the required dynamic range, still keeping the noise low enough to provide single photon detection at low signal intensities. In this work, we propose a very simple front-end (FE) solution based on an input PMOS transistor placed on the CMOS readout chip connected to the pixel detector. The FE is optimized for low-noise readout of X-ray photons at low intensities and characterized by a compression of the gain when the signal intensity increases. The PMOSFET is operated in the triode regime and its current is fed into a current-readout filter through a resistor. Larger is the transistor signal, larger is the voltage drop on the resistor which push the transistor to operate more in triode regime at lower gain, producing a compression in the overall FE response. The FE working principle and the first experimental results obtained with a first prototype realized in the 130 nm IBM technology are here presented.