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Disturbs on analog output (RGB) using ADV8005

Good morning, we are observing RGB output from ADV8005, processor is configured for PAL format. As an input we're using a reference video generator. In the image you see two of the color signal when using 75% color bar reference image (left) and a full magenta image (right). The blue trace in both images is our Green signal, as you can see it shows an important disturb (more than 100mV at the peak, absolute value) exactly on specific color changes (first image: between the second and third bar; second image: at the very beginning of the image).

We did a few tests to exclude it is a problem on the analog section or the output circuitry. It seems related to digital elaboration before the output encoder (we use the SD encoder on DAC 4,5, and 6). Did you have any guess on what can cause this behavior?

thanks

Fabrizio

  • Hi Fabrizio

    To me this looks like cross talk between traces or noise coupling via the power rails.  Looking at the first image during the active video region the green channel spikes positive on the falling edge of the color channel.  Also there is s small positive and negative spike in these channels indicating something else is coupling into the traces.  

    The second image shows a negative spike on the rising edge of the color channel which corelates with the first image.

    I would need to review the schematic and layout to come up with any specific suggestions.

  • Thanks for answer, we had an investigation on the points you mention and discovered that those disturbs are present even cutting the connection from the output DAC to output BNC (we had circuitry here with series resistors and we removed them); we used also register 0xE400 to enable just one of the output DAC, and also this intervention does not have an effect on the disturbs.

    We however had an improvement in the end: we put to 0 register 0xe482 bit 7 (slope_en: SD active video edge control); it was set in the previous tests. The disturb in the second image diminished. Other are unaffected. Based on this how do you think we can make the image better?

    thanks

  • 1) Do you have a solid ground plane under the device?

    2) Is the scope ground connection close to where the probe tips are attached?

    It still looks like a cross talk issue.  Without looking at the schematic and layout it is hard to come up with more suggestions.

  • 1) no we don't

    2) scope ground connection is just a few millimeters away from the point where the probe tips are attaccched.

    We know we probably can do better with HW, we have a question on another point. If I turn off DAC5-6 I don't have a video signal on R and B channel, yet the disturb on color bars is still present. How can it be?
    Thanks

  • 1) Are there any input signals running close to the output signals where cross coupling can occur?  Cables, traces, connectors?

    2) A while back I had a video circuit up and running with what looked like power line frequency noise injected into it.  It turned the florescent light in my office was generating enough EMI to couple into the circuit board.  You might just try turning off anything like a florescent lamp.  This is begin to look like power line noise injection.