Mitigation of bandwidth limitation induced crosstalk on Athena's WFI

Abstract

The wide field imager (WFI) for the ATHENA mission will enable the spectroscopic investigation of solid angles up to 40’ in the x-ray regime between 0.2 keV to 15 keV. The required sensors are planed as DEPFET (DEpleted P-channel field-effect transistor) active pixel matrices. These are readout by custom-designed ASICs called VERITAS in a rolling shutter mode. After column parallel processing of the analog signals row-by-row, the resulting pulse heights are multiplexed onto a differential analog line pair. Thus a transmission undisturbed from external influences is achieved. But successive signals can be affected by the limited rise-time of the VERITAS output stage, depending on the signal height, the multiplexing time and the cable length. In order to investigate the impact on the measurements a dedicated cable harness is used during the characterization of a prototype device. The characterizations are performed using either test pulses applied to the VERITAS or with a 55Fe calibration source illuminating the whole sensor. Different cable lengths and multiplexing times are tested to determine the influences. In order to deal with this effect a crosstalk correction algorithm is implemented for the analysis of the photon data. The determined crosstalk factors range up to 2.8% for the longest measured cablings of 2 m with the shortest multiplexing time. Using shorter cables and longer multiplexing times these factors can be reduced to below 1%. In all cases the accompanying effects in the data analysis can be corrected using the developed algorithm.