@Klaus I would avoid using the caps if I could find a better solution. I am sorry but I don't understand what you mean clock_return. Do you mean the negative edge? The clock that goes to the DAC is 180 to the one that drives the logic (and samples). I've also tried 90 and 270 degrees offset with not much success.
@FvM this is what I've been doing so far, but i consider it to be an elegant solution. I am getting the differential clock from the SMAs on the ML509. Ideally I'd like to run everything off the header rows. Plus, it frustrates me that I don't know what's wrong.
I will come back with the oscilloscope grabs you suggested later today.
Thank you both for your input.