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LVPECL and floating inputs

Discussion in 'Electronic Design' started by samiam, Jun 1, 2007.

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  1. samiam

    samiam Guest

    Software guy here working on a simple project thats becoming complicated

    Please imagine the following scenario:
    device with lvpecl inputs. Possibility that the inputs may float (left
    unconnected). I reckon I should force the inputs to a default level,

    If this were a TTL device, I would tie the input to vcc via a pullup
    or pull down. But its a differential - so I guess I need a voltage
    divider to balance the two inputs such that theres a difference of say
    100mv between the two

    How will this affect things when an input IS connected?

    I know what you are thinking - is this dude dense or what?
    I am going to go down to the basement and fish out my ac/dc analysis
    book and read up on thevenin,superposition,mesh networks and various
    ways of analyzing circuits.

    Its been awhile

    Any pointers would help
  2. Jim Thompson

    Jim Thompson Guest

    Differential? Read the data sheet. Probably tie one input high to
    keep the current mirrors under it happy, the other input low. But
    it'll depend on the exact circuit.

    ...Jim Thompson
  3. John Larkin

    John Larkin Guest

    You could thevenin terminate them to slightly different voltages,
    offset in your preferred direction, both near the low end of the
    common-mode range. That should work OK.

    What's the receiver device?

  4. samiam

    samiam Guest

    What's the receiver device?

    SY100EP15V Mux
  5. John Larkin

    John Larkin Guest

    That part has weak pullup/pulldowns so that it goes to a known state,
    equivalent to input low, if the inputs are truly open. But if you add
    your own terminations they will overpower those pulls, so you should
    make sure they add a small offset if you want the input to be in a
    known state when the signal source is gone.

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