ASTEROID: A 64 channel ASIC for source follower readout of DEPFET arrays for X-ray astronomy

The 64 channel ASTEROID ASIC has been developed to readout DEPFET Macro-Pixel Arrays operated in source follower mode. In particular ASTEROID will be coupled with the DEPFET Macro-Pixel sensors designed for the X-ray astronomy application BepiColombo. This is a satellite-based mission that requires a detector system with high speed readout, high energy resolution and radiation hardness properties. In the source follower operating mode of the DEPFETs, the front-end electronics is AC-coupled. This allows easy coping with non homogeneity of the pixel matrix and 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. This is the time-limited optimum filter for white series noise, which is dominant at the foreseen readout speed. The ASIC is realized in the 0.35 mu M 3.3 V AMS CMOS technology. The outputs of the 64 analog channels are multiplexed to one serial output with a speed up to 20 MHz. Thanks to the new multiplexer architecture adopted, ASTEROID is the only ASIC that allows window-mode readout of the pixel matrices, i.e. that allows to address selectively arbitrary sub-areas of the pixel array or even to readout different sub-areas at different speeds. In order to fully operate the 64 channels in parallel, the ASIC contains a digital section that generates the timing signals for the analog circuits. This digital section is based on SEU-immune dual port memory cells. ASTEROID has been tested for the first time with a demonstrator 64 x 64 macro-pixel Matrix of the size of 3.2 X 3.2 cm(2). The energy resolution measured on the Mn - k alpha peak of Fe-55 is of 124 eV with a pixel processing time of 4.8 mu s at -80 degrees C. This excellent result, never achieved before with source follower DEPFET at this readout speed, perfectly matches the requirements of BepiColombo Mission. (C) 2009 Elsevier B.V. All rights reserved.