Low frequency noise?

Dear Colin,

I am performing measurements with the oadc on XX target (not the multi-target victim board) and seem to be getting a lot of low frequency noise. In the attached picture, you can see three overlayed traces.

Any other ideas what might have caused this? I have also tried the following two setups without observing the same low frequency noise.
picoscope sine generator --> measured by oadc
XX target --> measured by picoscope

Also, what is the impedance of the oadc? At this page, it is stated to be 50 ohm or 6k ohm selectable by a jumper. Which jumper is this?

Look forward to your thoughts, thanks!


Sorry on the delay - was away for a few days and then just catching up now! Anyway one quick thing to check is trying to adjust the ‘fine phase offset’. Try extended ranges of -255 or 255. Let me know if that changes anything…


I tried shifting the phase, but the problem persists.
The DCM/ CLKGEN PLLs are locked and the openADC is sampling at the expected, correct frequency

It seems to be a DCM issue as I bypass the DCM and feed the extclk to ADC directly, then this low frequency noise disappears.


I’ll have to see if I can do some sort of measurements on the frequency coming out of the system out of interest… maybe the DCM block is introducing some noise in terms of frequency/phase errors…