ASTEROID: a new 64 channel ASIC for source follower based readout of the MIXS DEPFET arrays on BepiColombo

ASTEROID is a new 64 channel ASIC developed to read out a DEPFET macropixel array. This sensor will be part of the MIXS instrument that will be used in the ESA's BepiColombo Mercury exploration mission. The detector will operate in an environment with high density of solar proton flux that will cause severe radiation damage and an increase of leakage current during mission lifetime. Given the relatively high value of the leakage current, a high speed readout and a proper cooling of the detector is needed in order to achieve the required energy resolution. The contribution of the readout electronics to the noise should be smaller than 10 electrons r.m.s. The foreseen pixel arrays are optimized for source follower readout. In this operating mode of the DEPFETs, the readout electronics is AC-coupled. This allows easy coping with non homogeneity of the pixel matrix and - most of all - an easy compensation of threshold voltage shifts of the DEPFET devices due to radiation damage. In order to achieve the low noise value required at short processing time, ASTEROID implements a trapezoidal weighting function, which represents the time-limited optimum filter for white series noise. This is the major electronics noise source at the foreseen readout speed. Measurements on a first prototype of the ASIC have shown the very satisfactory resolution of 147 eV on Mn-K alpha peak of Fe-55 with a readout time of approximately 1.2 mu s, that would lead, considering the settling time of the DEPFET, to a total processing time of 4.2 mu s. We present the experimental results of a first prototype that includes 4 analog channels and a new digital section for the generation of the timing signals of the circuit. This digital section is based on radiation-hardened dual-port memory cells and registers and has already been implemented to serve all the 64 channels foreseen for the final version of the ASIC.