For AD9643, AD9613, AD6649, or AD6643 Family Evaluation Board and HSC-ADC-EVALCZ, how to configure the Internal AD9523 to configure SPI controller to Generate 250MSPS Sample Rate
I would recommend first downloading and installing the software for the AD9523 here: http://www.analog.com/media/en/evaluation-boards-kits/evaluation-software/Software_AD9523_AD9424.zip
I would configure the device as desired using the AD9523 GUI. The GUI can then be used to output a file with the desired SPI settings. Once you have these settings download and install SPIController from here if you don't already have it: ftp://ftp.analog.com/pub/adispi/A2DComponents/Install/SPIController_Setup.exe
Open SPI controller and select the AD9523spiR03.cfg config file. Use the SPI settings list generated from the AD9523 GUI and program them into the AD9523 on the ADC evaluation board. Once you have all the settings loaded you should be able to run the AD9523. I did notice on the product page on the website that the AD9523 is now not recommended for new designs so I would keep that in mind.
I'd like to add to the response just posted: The AD9523 output frequency is a function of the VCXO frequency used on the ADC evaluation board. If the VCXO frequency does not have a simple integer relationship to 250 MHz, they you probably won't be able to get exactly 250 MHz. On the other hand, if you have a 25, 50, or 100 MHz VCO, you should be fine.
What is the current output frequency of the existing setup?