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ADV7401 video drifting.  Rising GND reference?

Question asked by KevinW on Mar 20, 2015
Latest reply on Apr 13, 2015 by KevinW

Hi,

 

I am observing odd behaviour when passing video through my ADV7401 device.  I suspect it has to do with grounding but I would like to ping some ideas from this community since it has helped me a great deal in the past.

 

I have a ADV7401 device in my design configured to accept video on AIN 1, 2, 3 and it pass it through over the 24bit output to an FPGA.

 

Problem:

When I place an osc probe on the data line at pin 41 (P3, second LSB) I can see evidence of video but over time (~5min) the video diminishes, not in amplitude, but in value.  The vertical lines on my scope between the syncs are such that I am initially seeing many transitions but as the ~5min mark approaches, the value lessens such that I'm seeing less frequent transitions until finally, there are none, and I see a flat line at 0V.  I ran this test multiple times and observed consistent behaviour.

 

I ran a third test, this time putting my probe on a MSB (P9) and saw that the video disappeared quickly (just over ~2min).  I then probed an LSB (P3 again) and saw video for the remaining ~3min.

 

Can anyone suggest why I am seeing such odd behaviour?  It would seem like my grounding reference is rising.

 

Other test points I can probe to help narrow in on the root cause?

 

I am only using AIN 1, 2, 3 and SOG.  All other video inputs (AIN 4- AIN 12, SOY) are not connected (floating).  Could this be an issue?

 

The video input is VGA at a resolution of 720x576 50Hz interlaced.  Should the FB pin be pulled high?  Low?  We are not using SCART.

 

I can provide more details as needed.

 

Thanks in advance for your help.

 

Kevin

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