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

We present the complete 64 channel version of the ASTEROID ASIC, developed to readout DEPFET Macro-Pixel Arrays operated in source follower mode. These sensors have been designed for the X-Ray Astronomy applications Simbol-X and BepiColombo. Both are satellite based missions that require a detector system with high speed readout, high energy resolution and radiation hardness properties. The foreseen baseline 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 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 filtering scheme substitutes the Multi-Correlated-Double-Sampling used so far by CAMEX ASICs and represents the time-limited optimum filter for white series noise, which is dominant at the foreseen readout speed. Measurements on single pixels had shown that an electronics noise as low as of about 8 electrons r.m.s. is achievable at room temperature with a total processing time of 4µs. In this work, preliminary measurements on the complete 64 channel readout ASIC coupled with a 64×64 DEPFET matrix have confirmed this noise performance. This result fits the requirements both of BepiColombo and Simbol-X. Also the other parameters of the ASIC have been tested on the 64 channel version. 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. The outputs of the 64 analog channels are multiplexed to one serial output with a speed up to 20MHz. Thanks to the new multiplexer architecture adopted, ASTEROID will be 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.