I am currently working on a function generator supplied and controlled by USB. I have lots of problems with que quality of the signal. I already made a double sided board just with the DDS and its MCLK reference but i dont know if the layout is ok. My objective is to generate frequencies till 3 MHz sinewave with the DDS with the 25 MHz MCLK (i know the squarewave can have higher frequencies but when i specify a function generator frequency is always the sinewave reference - the other waves must have lower frequencies). The problem is that the signal is floating a bit and i have noise on top of it (along all the range of frequencies) (i tried to put 4 caps decoupling VDD-AGND and another 4 decoupling VDD-DGND - 10uF, 1uF, 10nF and 100nF - but there is still lots of noise and floating) and when i reach the higher frequencies i can see the sampling points ("steps"). This higher frequency problem can be corrected with a low pass-filter? or it is difficult to do it?
Now about layout... can you just look at the 3 images i attached and see if i have something wrong? (i have 2 groung planes - one for AGND and other for DGND and i join then on the bottom plane with a 0 ohms resistor - is it correct?)