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LMH1297: VIN and VDD_CDR Combined or Separate?

Randy May
Prodigy 70 points
Part Number: LMH1297

Greetings,

I have a card with 4 LMH1297s on it.  Each channel is exclusively an Input or an Output, so SDI_OUT is not used.  There is some performance degradation when I run a TX adjacent to an RX through a long channel (approx -40db), and I'm looking to improve the next spin of the design to minimize the degradation.  Would it be beneficial to separate VIN and VDD_CDR onto different planes which I tied together a common point somewhere else on the board?  Our current design has these pins sharing the same 2.5V plane, which is isolated from the other LMH1297s through ferrite filters.  The EVB has these supplies on separate planes which are tied together at a header.

Thanks

  • 0
    TI__Guru* 93670 points

    Hi Randy,

    Please note equalizer uses 1.8V and thus this 2.5V supply is regulated. It is strongly recommended to do lab experiment to find the root cause before doing a new pcb. You don't want to go through a new board and then realize there is no improvement. For example, please unsolder the cable driver BNC connector, cut trace going to the connector, and then terminate with 75ohm to GND. Do you see the problem? If yes you found the root cause. Else, please shorten or cut additional cable driver trace and check again. I think through this process you would be able to find where the problem is coming from and it may end up being related to the power supply noise from adjacent device as you may have suspected.

    Regards,,nasser