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LMH1297EVM: Diifferential Inputs and Outputs in parallel

Roger Berk
Prodigy 90 points
Part Number: LMH1297EVM
Other Parts Discussed in Thread: LMH1297

Hello !

I would like to connect the differential Input IN0+-  and output  OUT0 +- ports of the lmh1297 in parallel to emulate the sdi IO functionality of the 75 ohm port on the other side of the board.

In the EQ mode ( set by J10 ) the differential IN0 +- are automatically disabled.

In the CD mode ( set by J10 ) both the IN0 and OUT0 are enabled, but J11 can disable the OUT0 port.

Will these disabled ports become high impedance when disabled. I am concerned about a double termination if I parallel the ports.

The circuitry that will connect to the lmh1297 can be either an input or an output on a single differential pair, and will only change at power up.

I hope to use a switch or software to set the card for input or output without the need for external cabling changes or multiplexers.

 

 

  • 0
    TI__Guru* 93670 points
    Hi Roger,
    I think i understand what you are trying to achieve here. What is the application for this?

    The way you have it, there would be double termination - as you noted. Based on your receiver , it might be able to handle the reduced output swing level(LMH1297 amplitude level is programmable). Also, on LMH1297 side there is a retimer that would clean up the jitter due to this mis-match. The other thing to consider is the stub or antenna effect of the un-used 100 ohm differential pair. So from signal integrity point of view especially at 12G data rate, this doesn't look promising.

    Another idea is to layout AC coupling caps such that it would go to either input of the LMH1297 or output of the LMH1297 device. However, this means you would have two hardware - if this is acceptable.

    Regards,,nasser
  • 0 in reply to Nasser Mohammadi
    Prodigy 90 points

    Thanks for your help Nasser.

    Optical SDI modules are available in dual TX, dual RX and RX + TX configurations. I was hoping to make software or switch configurations  to avoid using external crosspoints or physical coax connections to accommodate all of the possibilities of modules the customers could select .

    I was hoping that the disabled differential pair would not cause trouble, but as you say, the potential of an open pair and double terminations would probably not be the best way to handle things .

    Thanks for the sanity check and insights . Roger