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SN75DP159: Drop in HDMI amplitude when GND directly connected to earth

Sebastian Ciesluk80
Expert 1121 points
Part Number: SN75DP159


Tool/software:

Hi , we are trying pre compliance tests using setup as attached (for simplicity only one data pair is shown) .

Many monitors available on the market, directly short shield to gnd and to earth (of power plug). As long as we do not create such short we have great eye ( signal shape) as per attached waves ( we use HDMI analyzer as sink , where we have EDID emulation and no link to earth).

Once we short gnd to earth (dotted line on the diagram) signal drops to the level where eye is not acceptable.

If gnd and earth are not short the potential difference between them is -3V and it is due to 3.3V pull up of the HDMI data lines. We have tried to cut the lines between our device and the analyzer and introduce external pull up what showed that potential difference disappears once pull up is removed. It almost looks like ground loop through the SN75DP159 but we are not sure how. Have you seen such behaviour? Are you able to comment on what could be the reason?  

Thanks in advance

Sebastian

  • 0
    TI__Guru**** 180881 points

    Sebastian

    For the supply that is providing the 3.3V to the external pullup, does it have the same ground as the DP159 PCB ground? If that 3.3V source has a separate ground, then it could create multiple conductive paths to ground which I do agree with you that it could be ground loop issue. But this is not an issue I have seen when I was doing the DP159 compliance testing. 

    Any chance you could share your schematic and layout as well?

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Expert 1121 points

    Hello David and thanks for picking this up.

    The 3.3V pull up is internal to the analyzer which is battery powered. Analyzer ground is linked to DP159 through HDMI cable hence grounds are shorted. I can not disconnect this cable ground as it is essential for edid. Could we move to email conversation then I can send schematics and layout ?

    Thanks Sebastian

  • +1 in reply to Sebastian Ciesluk80
    TI__Guru**** 180881 points

    Sebastian

    Would you please accept my friendship request? You can then send me the schematic and layout in a private E2E message.

    Thanks

    David