Hi David,
Keeping adjacent 3.3V CMOS single-ended inputs from coupling is always challenging with any part, and we have had customers who have been happy with the AD9518 performance when driven by two single-ended inputs.
The keys to getting this are:
1. Ensuring that the SE clocks are routed for maximum spacing on your board.
2. Ensuring that proper impedance matching techniques are followed.
You can try reducing the input amplitude, or turning off the unused input receiver. However, the latter fix won't help if there is board-level coupling.
The best way to minimize coupling and maximize performance is to reduce the amplitude while maintaining the input slew rate.
I hope this helps.
-Paul Kern
