I am seeing 1-4Vpp xMHz frequency sine wave superimposed onto all of my injected signals on my own test board. I have tried injecting a pulse of different widths at 5MHz, a 10MHz sine and square wave, and DC values 4V and 0.5V. The amplitude and frequency of the superimposed sine is consistent for each injected signal but varies across signals. I have 2 identical boards - one is able to see the DC values with the sinusoid imposed but one shows garbage for all DC values.
My set up is BNC connector - FDA - ADC. When grounded or left floating at the BNC connector, I see a full range sinusoid. Measuring the injected signal at the output of the FDA shows a clean signal. If I ground the lines between the FDA and ADC with a floating BNC cable, I don't see any noise.
The signal integrity of the JESD lines are good and I am sure I am interpreting the data from the ADC output correctly. All of the test signals (ADC, SERDES, link layer) and a stress toggle pattern for each data lane pass. Where could this be coming from?
