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MC10EL11 termination

Discussion in 'Electronic Design' started by [email protected], Sep 2, 2005.

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  1. Guest

    Hi. I am designing a board which uses MC10EL11, ECL fanout chips. After
    reading their data sheet and AND8020/D, I got a bit confused. Can
    anyone confirm that my termination plan will work?

    I will have 100 ohm resistor across pins 6 and 7.
    Also, I will have 220 ohm resistors across 5 and 1, 5 and 2, 5 and 3,
    and 5 and 4. Ideally I want to use 235 ohm ones, but I couldn't find
    such things.

    Thank you for your help.
     
  2. w2aew

    w2aew Guest

    The 100 ohm differential termination on the input (pins 6 and 7) will
    only properly terminate a fully differential signal (not a single-ended
    signal), but only if the device driving this input can effectively
    drive this differential load, and the input traces are either 50-ohm
    lines or a 100-ohm differential pair. So, without knowing what is
    driving this fanout buffer, we can't answer this first part properly.

    The pull-down resistors on the two sets of differential outputs will
    adequately bias the emitter follower outputs. However, they will not
    help to terminate a long PCB trace or transmission line. If the output
    traces are short (an inch or less), then you'll probably be OK.
    However, if the lines are longer than this, then you'll have to
    consider a different termination scheme to precent reflection problems.
    Very good results can be obtained by putting proper terminations at
    the far end of the lines (opposite end from the driver). Proper ECL
    terminations will present a 50ohm load that is terminated to a VCC-2V
    potential. This properly terminates the 50-ohm transmission line (to
    avoid reflections), and also presents a proper bias to the emitter
    follower outputs of the driver. A split termination can be used on
    each line which presents the thevenin equivalent of 50ohm to VCC-2V.
    For typical 5V ECL/PECL applications, a split termination is made by
    putting an 82ohm resistor from VCC to the line, and then a 130ohm
    resistor from the line to VEE. If you are routing the signals
    differentially, then other delta or Y style terminations can be used
    which don't draw standing power like the spit termination does.

    Lots of good information on proper handling of ECL/PECL circuits can be
    found on Freescale Semicondutor's website (formally On Semiconductor,
    formally Motorola Semiconductor).
     
  3. Guest

    Thank you. I will drive this device with fully differential ECL signals
    and 100 ohm differential pair input traces.
     
  4. w2aew

    w2aew Guest

    OK. You can save the standing current of the split terminations by
    using either a 'delta' or 'wye' differential termination. These
    terminations are to be placed at the receiving end of the line, not the
    driven end. Thus, you'll have a termination at pins 5&6, and then at
    the ends of the lines driven by 1&2, and 3&4.

    Assuming VCC-VEE=5V:
    The delta termination consists of a 120-ohm resistor connected between
    the differential lines, and a 270-ohm resistor connected from each line
    to VEE.

    The wye termination consists of two 50-ohm resistors in series,
    connected between the differential lines, and a 110-ohm resistor
    connected from the junction of the two 50-ohm resistors to VEE.

    Each of these terminations does a nice job of approximating the ideal
    50-ohm to VCC-2V termination that ECL likes to see. NOTE - if your
    power supply (VCC-VEE) does not equal 5.2V, then you'll have to
    recalculate the resistors slightly.

    I hope this helps!
     
  5. w2aew

    w2aew Guest

    Correction:
    The values shown are valid for VCC-VEE=5.2V (not 5V).
     
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