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LVPECL termination

Discussion in 'Electronic Design' started by Tom Derham, Nov 19, 2003.

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  1. Tom Derham

    Tom Derham Guest

    Normally LVPECL receivers expect termination to be 50 Ohm to Vtt (which is
    Vcc - 2V, or 1.3V for normal 3.3v lvpecl).

    However, I have seen a couple of evaluation boards (one of which I rather
    naively followed for one of my board designs) where both (differential)
    outputs are terminated into 50 ohm to ground.
    The signals are then used to drive the differential inputs to a DDS chip
    (high impedance) requiring essentially lvpecl levels (centred on 1.6v dc
    approx).

    Can anyone suggest what the effect of this difference in termination will
    have on the circuit?
    Is it correct / acceptable design practice?

    Many thanks

    Tom Derham
     
  2. John Larkin

    John Larkin Guest

    Well, the drivers will have to source about 50 ma when they're high,
    which does burn up a bit of power but is probably OK... check the
    driver spec to be sure. I'd personally go for a termination that
    wastes less power and makes the driver work less.

    In real life, with Vcc just 3.3, this should work, which is I suppose
    the definition of correct/acceptable.

    John
     
  3. I'd think the drivers would be working rather hard.
    I wouldn't let this go.

    There are a number of options here. I've used a split-terminator with
    83ohms to ground and 125ohms to +3.3V quite successfully. In your case
    this is simply a value change and the addition of the upper resistor.
     
  4. Daniel Lang

    Daniel Lang Guest

    At 3.3 volts, the high level current will be about 50 mA which is near
    the absolute maximum for ECL and wastes power.

    I would use a Y termination at the receiver using 50 ohms from each of
    the lines to a third 50 ohm resistor to ground.

    Daniel Lang
     
  5. Mac

    Mac Guest

    I wouldn't think it's acceptable at all. You should check the data sheets
    for both the receiver and driver to see if you are violating anything.

    A couple of the receivers I've seen have a weak drive on one of the pins,
    which would be enough to set the common mode voltatge. In that case, you
    can just put 100 Ohms across the receive terminals.

    Another option is to use 50 Ohms from each receive terminal to AC ground
    (a capacitor). This has the advantage of greatly attenuating any common mode
    signal that may be present. But something always has to set the common
    mode voltage. Either the driver or receiver, or a pullup/pulldown network.

    In your case, what you have to hope for is that the driver will be happy
    providing the fairly hefty DC currents you probably need to keep the
    receiver in its rated common-mode voltage.

    Mac
    --
     
  6. Fred Bloggs

    Fred Bloggs Guest

    You saw no such thing- your reverse engineering, aka copying , is in
    error...
     
  7. John Larkin

    John Larkin Guest

    The logic here is ECL: open emitter outputs. This *must* have a
    substantial DC pulldown to work: the driver can only pull up.

    50 ma is a tad high, but you'd need at least 20 to work properly here.


    John
     
  8. Mac

    Mac Guest

    Well, it's LVPECL, not ECL. And a lot of LVPECL stuff is really just
    differential. And as far as I know, LVPECL doesn't mean open-emitter, it
    just means emitter-coupled. That is, there's no reason the pulldowns
    couldn't be on the driver. But the bottom line is that he has to look at
    the datasheet for the driver and receiver to see if he is violating
    anything.

    FWIW, I made a cursory survey of a few LVPECL parts, and the ones I saw
    actually can drive 50 Ohms to ground.

    Mac
    --
     
  9. John Larkin

    John Larkin Guest


    LV means low voltage. PECL means ECL that is operated with shifted
    supplies, ie Vee = ground. If you take any old ECL part and wire its
    Vcc and Vee pins to V+ and ground respectively, then you call it PECL.
    It's still open emitter. I don't know of any ECL (or 'LVPECL') parts
    that have internal pulldowns. LVDS drivers, of course, pull up and
    down.

    There may be some parts that advertise 'LVPECL output levels' and do
    drive up and down, but they're not actually ECL parts. Anybody know of
    any?
    Right. 50 mA isn't really a lot for these brutes.

    John
     
  10. Mac

    Mac Guest

    Well, there is a sizeable minority who say the 'P' in PECL is for
    "pseudo," not "positive." I'm not one of that minority, mind you.
    Certainly receivers.

    The xilinx virtex II fpga can use some kind of "PECL" outputs. I've never
    looked into it because I never needed to, but I bet they're not really
    PECL. I mean, I assume there is only one die in the package, and it is
    130nm CMOS. Can they mix bipolar and CMOS on one die?
    Mac
    --
     
  11. John Larkin

    John Larkin Guest


    It used to be 'pseudo', as in the old Motorola books. But some PR type
    must have decided that sounded funny, so it's been rehabilitated to
    'positive'. I still call it 'pseudo'. I did finally stop calling
    capacitors 'condensers', though.


    Receivers drive down? I thought receivers, er, received.

    John
     
  12. Mac

    Mac Guest

    Aha! Thanks for clearing that up for me. I have never heard anyone call a
    capacitor a condenser. ;-)
    Well, the only thing I can think of is that I was responding to the "not
    actually ECL" part of your question. But I concede that that part of my
    reply makes no sense.

    I mean, a lot of differential receivers do have weak pulldowns. And they
    can detect when the signal is not otherwise driven. When they detect that
    condition, they drive their outputs to a logic low. But this isn't what I
    was talking about, I don't think.


    regards,
    Mac
     
  13. John Larkin

    John Larkin Guest

    Ah, a youngster you are (in Yoda-speak.) But there are still a few
    cars and motorcycles with 'points', and they still have 'condensers'.

    Somewhere around here I have a really handy CPS-to-Hertz nomograph...
    Yes! I'll post it to a.b.s.e.

    John
     
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