VELA: A fast DEPFET readout circuit for the NHXM mission

NHXM, under study by ASI (Agenzia Spaziale Italiana), is an X-ray observatory in the energy band between 0.5 and 80 keV and will have 3 telescopes dedicated to X-ray imaging with a field of view diameter of 12 arcmin and a focal length of 10 m. We report on the development of high-speed and low-noise readout of a monolithic array of DEPFET detector. The DEPFET based detectors, thanks to an intrinsic low anode capacitance, are suitable as low-energy detectors (from 0.5 to 10 keV) of the new NHXM telescope. The challenging requirements of the NHXM cameras regard the necessity to obtain images and spectra with nearly Fano-limited energy resolution with an absolute time resolution of about 100 µs. In order to exploit the speed capability of the DEPFET array, it has been developed a readout architecture based on the VELA circuit: a drain current readout configuration to implement an extremely fast readout (2 µs/row) and preserve the excellent noise performance of the detector. In the paper the foreseen maximum achievable frame-rate and the best energy resolution will be presented in order to assert the VELA suitability for X-ray imaging and spectroscopy.