Numerical simulation and validation of ATHENA/X-IFU/digital readout electronics

The X-IFU (X-rays Integral Field Unit), one of the two instruments of the Athena mission, is a cryogenic Xray spectrometer for high-spectral resolution imaging. The large array of 3840 detectors each composed of an absorber coupled to a Transition Edge Sensor(TES) will be operated with a bath temperature of 50 mK. This instrument is designed to provide a challenging energy resolution of 2.5 eV in the 0.2 to 7 keV range. The DRE (Digital Readout Electronics) drives the frequency multiplexed readout of the sensors and implements the feedback required to optimise the detection chain dynamic range. To comply with the instrument energy resolution requirement, the constraints on the detection chain sub-systems are very stringent (thermal stability, signal to noise ratio, linearity,...). This implies a strong optimisation effort during the design of the sub-system in order to both satisfy the performance requirements and to fit in the mass, volume and power allocations. We have developed a numerical simulator of the X-IFU detection chain in order to validate the architecture of the DRE. The simulator implements the contributions of the different detection chain elements in the overall instrument performance. The details of the DRE architecture are included in the simulator and we use it to validate the different design options.