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The Wide Field Imager (WFI) instrument of the ESA astrophysics mission ATHENA (Advanced Telescope for High Energy Astrophysics) will target the investigation in the X-ray energy range between 0.2 keV to 15 keV. The Camera Head (CH) comprises two detectors, the Large Detector Array (LDA) and the Fast Detector (FD), with respective readout timings of 2 ms and 80 µs. The LDA consists of four Large Detectors (LDs) where each one is composed of a matrix of 512 x 512 pixels of depleted p-channel field effect transistors (DEPFETs) whereas the Fast Detector is composed of 64 x 64 pixels with a pixel size of 130 µm. At the start of the WFI signal chain is the DEPFET Sensor in the CH, with which the incident photons interact to create a signal. It is read out by the (Application-Specific Integrated Circuit (ASICs) of the Front-End-Electronics (FEE) and further processed by the Detector Electronics (DE), until at the end of the signal chain, the Ground Support Equipment (GSE) is reached. Each interaction and sub-system add a significant noise contribution, leading to an overall degradation of the measurable energy resolution. The total WFI spectral performance hence depends on the spectral performance of the CH and the noise contributions from individual components along the signal chain. We present a detailed analysis that distinguishes the different noise terms, which sum up to the total noise of the system along the complete signal chain. As the performance of the signal chain is responsible for most of the critical requirements of WFI, individual contributions are either determined by specific stand-alone measurements in the laboratory, analysis, calculations or represent estimates of individual noise contributions to the entire system, as well as heritage data from previous missions. An allocation of the performance budget for the spectral resolution of both detectors of WFI is given for system engineering.
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