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LMH0346 to two or more LMH0307

Greg Cooper
Prodigy 50 points
Other Parts Discussed in Thread: LMH0346, LMH0307, DS25CP104A, LMH0394, LMH0395

I am designing a 3G distribution amplifier.  The LMH0346 has two outputs, but I want to fan (or parallel) the outputs on both to provide a one input, eight output distribution.  In order to achieve this I need to go out of the reclocker to two cable drivers.  How do I do this while maintaining my 100 ohm balanced line and termination?  Anyone have a schematic or an idea?  Thanks!

  • 0
    TI__Expert 4400 points

    From your description, it is not clear to me whether you are designing a 1:8 or 1:16 distribution amplifier using the LMH0346 SDI reclocker and LMH0307 SDI cable driver, so I will explain both scenarios below.

    If you only need 1:8 (and you do not care about the polarity of the outputs), then you can build this using only one LMH0346 and two LMH0307s.  The LMH0346 has two differential data outputs.  Each output can be connected to an LMH0307, and each LMH0307 provides four 75Ω single-ended outputs.  The two outputs from the LMH0346 can each be routed with 100Ω differential traces and each terminated with a 100Ω resistor at the inputs to the LMH0307s.

    If you need 1:16 (or 1:8 with all positive polarity outputs), then you need one LMH0346 and four LMH0307s.  To interface between the LMH0346 and the four LMH0307s, I would recommend using a crosspoint switch such as the DS25CP104A.  Using one DS25CP104A, you could either configure it as a 2:4 fanout and feed it both outputs from the LMH0346, or configure it as a 1:4 fanout and feed it only one of the outputs from the LMH0346.

    Gary Melchior

  • 0 in reply to Gary Melchior
    Prodigy 50 points

    Gary,


    Thank you for your answer.  You are correct, I must have overlooked the fact that the LMH0307 has the four outputs.  That will work, although I don't know how crucial it is to have the outputs all be in phase.  All signals will not be tied together with any kind of a timing circuit; they are all independent destinations.  I only need 8 outputs for now so this configuration will work.  Also here is another question.  Is there such thing as  "loop through" as it was described in analog video?  I will need to "loop" out of one board to another using my 75 ohm BNC connectors.  Can I use one of my equalizer outputs (LMH0394) to do this?  I'm not sure if this will affect the rest of the circuit's reliability though since it is being passed through without reclocking.

  • 0 in reply to Greg Cooper
    TI__Expert 4400 points

    You could use the LMH0395 equalizer (in place of the LMH0394) since it provides a second output which could be used for the loop through.

    If you need this loop through output to drive a 75Ω BNC, then you’ll also need to pass this signal through an SDI cable driver.

    Gary Melchior

  • 0 in reply to Gary Melchior
    Prodigy 50 points

    Gary,

    Thank you.  Great info.  I was wondering if there is another way to take that LMH0395 equalizer output to another smaller, cheaper connection to the input of the next board.  These boards will be next to one another, only 1" apart.  The cost of adding an additional cable driver and BNC's is adding up.  Otherwise I will do it this way. 


    Greg

  • 0 in reply to Greg Cooper
    TI__Expert 4400 points

    Greg,

    Since it is a short distance, you could probably send this signal differentially and avoid using an additional cable driver.  But I do not know if you will save on the cost by using some sort of high speed differential connector vs. using a BNC (or SMA or similar) coaxial cable connector.

    Gary Melchior