We have a problem with our AD9681-125EBZ board (01-039100, REV B). We are using it with the Digilent Nexys Video FPGA board. When sampling very low power signals, the ADC works as expected. But when sampling signals closer to the full scale, the noise floor raises significantly (together with the harmonics). Effectively we have at most 8 bits worth of signal and SNR close to 50 dB instead of the 74 dB specified in the datasheet. See the image at the bottom.
Unfortunately none of the above have seemed to improve (or worsen) the effect. Thus, we would greatly appreciate any comments/ideas on how to proceed and get results closer to the specified ones.
Thank you for your interest in AD9681, and for the thorough testing you have done.
What kind of bandpass (anti aliasing) filter are you using on the analog input signal? For the vast majority of signal generators, a good bandpass filter is essential for optimal performance. If you are not using a bandpass filter, this is the first thing to try. There is more discussion on this in AN-835, in the ANALOG SIGNAL FILTER section.
If you are already using a good bandpass filter, I have a few additional questions.
We threw a 5MHz low-pass filter that was laying around and it seems to have fixed the issue. Before we tested with anti-aliasing filter, but that didn't improve our results.
Thanks for your help.
That is great news!
I hope your project goes well. Take care.