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DAC5672: 2nd harmonic distortion

afarin AF
Prodigy 10 points
Part Number: DAC5672
Other Parts Discussed in Thread: CDCE62005

Hi Ti Team 

There is a high second harmonic at the output of DAC5672 .in the below picture DAC Sampling rate is 210MSPS and fdata=71MHz .as seen, the 2nd harmonic is 47dbm. Please suggest points to fix this problem.

we connect to the Agilent spectrum analyzer directly through the SMA connector cable and the input clock is fed through the CDCE62005 with an external reference clock of 90dbm SFDR.

 

The schematic is

and the data traces of DAC are matched length in PCB.

  • 0
    TI__Mastermind 26210 points

    Hi Afarin,

    Would it be possible to see the layout section on the schematic portion you supplied above?

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 10 points

    thank you very much for the replay

  • 0 in reply to afarin AF
    TI__Mastermind 26210 points

    Hi Afarin,

    the issue is that these traces are not length matched. Even the grounds need to matched as well and I see the vias to ground are not matched, which will make a difference in the balancing of the DAC output.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 10 points

    Thanks a lot for all your help. Is there any way to compensate for these issues (the trace lengths are equal as below but vias are not mathced)?

    the below picture shows the signal length highlighted in blue. 

  • 0 in reply to afarin AF
    TI__Mastermind 26210 points

    Hi Afarin,

    I would look more carefully as the two diff traces come from the output of the DAC to R157. The corner bends on the traces can make it seem even. I would measure the length of those two traces to see if equal.

    You can try to force an imbalance, which will rebalance the input. On one side or the other you can put a 1pF-2pF cap to ground somewhere long the DAC output as it goes into the transformer. You will probably need to play with it a bit to get it right. Also, one way to find the sensitivity is to take a set of tweezers and probe around the traces, while looking at the output spec analyzer. If the HD2 improves you now know where the sensitivity is.

    Regards,

    Rob