Hi Govind,
Thanks for the schematic that you emailed me. I don't see anything big standing out, but I have some comments:
1. Wrong Period: I had noticed that the CLKOUT had a 146Hz repetition rate! Your problem is not just the "disturbance" on the high side but in fact completely wrong waveform. Repetition rate should either be the same as your INCLK (8MHz) or some multiple of it.
a) Can you confirm that RESET_bar (CIS_0EB) is not being toggled at this rate which it should not be. Can you please confirm?
2. Schematic dot missing: Please make sure the VR pin is tied to C29 (schematic dot is missing). Also, please confirm that DVB pin is only tied to C34 (and not connected to 3.3V). I'm hoping that your board layout does not get confused by this drafting issue?
3. CLKOUT Probing things to try: Did you try the experiments (with regard to the way the CLKOUT is being probed)?
a) What is nature of the oscilloscope probe you are using (e.g. normal 10Mohm 10x scope probe?, etc.)
4. Waveform Capture(s): Can you capture the following waveforms simultaneouslyand share?
INCLK+, CLKOUT, RESET_bar
5. Missing large (4.7uF) caps: There are several 4.7uF caps (called for in the datasheet) that are missing on the following pins:
VA, VCLP, VD
You have shown 10uF caps on your schematic page 3 and I wonder if these are close enough to the LM98714 (they should be) to be effective?
I would place the 4.7uF caps noted next to the pins as well as on 3.3VDVAC and 3.3VADC.
6. Ground Separation not needed: You have kept AGND and DGND separate on your schematic and I think you use 0ohm resistors (R14 and R15) to tie them together probably near the LDO? Our general guidelines is to not separate the analog and digital grounds and have them tied to each other near the device. Is this something that you can implement and try (or at least use a scope to verify that the two grounds next to the LM98714 do not have large / fast transients)?
Please let me know your findings from the list above.
Regards,
Hooman