Mixed signal pnCCD readout ASIC for the future X-Ray astronomy mission eROSITA

For the eROSITA X-ray telescope (extended Roentgen Survey with an imaging telescope array) aboard the new Spectrum-Roentgen-Gamma satellite an improved type of the pnCCD used for one of XMM-Newton cameras will be used. The eROSITA X-ray telescope instrument consists of seven mirror modules (Wolter-I optics) each having its own 384times384 pixel frame store pnCCD detector in the focal plane. The mission time of eROSITA including the all-sky survey as well as pointed observations will be in total about 5 years. For low noise and high speed performance the CCD is a column parallel device, therefore all 384 CCD channels are equipped with an on-chip JFET operated in source follower mode which are read out in parallel. The corresponding readout ASIC named CAMEX consists of 128 parallel readout channels each of which is equipped with an eight fold correlated double sampling filter. To fine tune the filter weighting function and therefore the noise and speed performance, we included a programmable RC low pass for bandwidth limitation. Each of the seven eROSITA modules uses three CAMEX ASICs. So a total of 21 readout ASICs are utilized. To make the module design easier and more robust against process and environment parameters these new chips feature integrated bias current DACs (digital analog converters) and include a digital sequencer for switch sequencing control. With the DACs the bias currents of the operational amplifiers can be set for each chip individually. The settings of switch control, gain, bandwidth, DAC and other operation modes are stored in digital registers. Programming is done over a simple serial interface. Noise performance achieved together with the pnCCD reaches 2 electrons ENC (equivalent noise charge) at a dynamic range of 2000 electrons.