AD9915 can be used with directly 2500 MHz clock or 2500 MHz system clock which
can be obtained from 100 MHz external clock with multiplier via PLL enabled.
Which situation is useful for best performance AD9915?
If you look in the datasheet at figures 15 and 19, you can see the phase noise
difference with a good external source vs. the internal PLL. If you have a high
quality external source, that will always be the best solution. So the best
phase noise performance is obtained by an external 2500MHz source vs the
internal PLL. For an external PLL solution, you might check out the AD9525.
It's our best phase noise performance to date.