DE2-115 and Terasic High Speed AD/DA daughter card

You should be driving ADA_OE LOW and ADA_SPI_CS HIGH.

If you've nothing connected it's possibly the noise you're seeing. Try connecting the ADC input signal (ADC_A_IN) to ground. Having said that, the board has 49R to ground anyway. That's not likely to be generating much noise across it.

So, I think it's more likely that the data output signals on the AD9254 are tri-stated and it's those lines that (may) have the noise on. Driving ADA_OE low (and ensuring the ADC has a clock) should be enough to get something sensible out of it.

Cheers,

Alex