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DS92LX2121/2122 Signaling Issue

Adrian Soltero
Intellectual 1135 points
Other Parts Discussed in Thread: DS92LX2121, DS92LX2122

Hi,

 

I am using the DS92LX2121 and DS92LX2122 devices to signal a LCD monitor.  The LCD monitor is expericing flickering in noisy envrionments and a DC offset has been seen on the SERDES channel.  We believe it is caused by an impedance mismatch or improper termination.  Do you have any schematic or layout recommendations to check for the possible cause of problems?

 

Thanks,

Adrian

  • 0
    TI__Guru* 93670 points

    Hi Adrian,

    You mentioned the LCD monitor is experiencing flickering in noisy environment. Am i correct in saying that there is a mode where there is no flickering and all is well? Please let us know under what condition we are running flicker free. Secondly, when there is flickering are we losing lock on the receiver side?

    With respect to your question and potential ideas, please note below:

    1). It is possible that because of the reflections or impedance mismatch you are seeing this low frequency common mode modulation. I noticed there is some type of protection device(i.e D54) would you see the same problem if we cut off the trace to this part?

    2). Also, it could be possible that perhaps there maybe power supply noise. I understand there is LC filtering but using a lab supply to rule this out could be a good idea.

    3). Please make sure we use differential scope probe since a poor gnd probe may show a similar problem to the scope picture with some type of common mode modulation.

    4). Please note gerber/schematic for Channel Link II:

    http://www.ti.com/lit/ug/snlu006/snlu006.pdf

    5). Please note Channel Link II Design Review section, almost at the end of the document noted below:

    http://www.national.com/assets/en/boards/national_channel_link_ii_design_guide.pdf

    6). Please note scope shots, attached file, of different points on the channel link evaluation board:

  • 0 in reply to Nasser Mohammadi
    TI__Intellectual 1135 points

    Hi Nasser,

     

    The screen flickers during radiated immunity tests, CRFI and EFT.  The problem could be caused by a possible ground bounce issue, which is leading to a common mode violation on the SERDES link between the controller where the serializer is located and where the deserializer is located.  How sensitive is the LVDS link to common mode differences?  Only about 60 mV of DC difference is seen but the AC difference still needs to be checked that might be coming from EFT or CRFI pulses.  Any suggestions?

     

    Thanks,

    Adrian

     

  • 0 in reply to Adrian Soltero
    TI__Guru* 93670 points

    Hi Adrian,

    Normally devices are AC coupled so common mode voltage should be isolated from one another. But if for some reason the common mode voltage is being violated or is being modulated by a low frequency signal then all of our bets are off.

    If this issue is occuring just during radiation test, could magnetic coupling be inducing theses bit errors?  Have they tried to shield the boards ?

    Regards,,nasser