Re: [SI-LIST] : Timing on a large board

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Fri Oct 06 2000 - 14:20:41 PDT


The 50-Ohm info:

Pages 3-90, 3-91, 3-92 of MC10125 datasheet, lool at the bottom of the table row
"switching times". The measurement condition is with 50 Ohm load.

If it is not point-to-point, use Thevenin split - it will save you drive current (cuts
it in half). Though with 100 Ohm and this driver you may be able to use AC termination. I
like 220 - 300pF cap value; seems to be optimal. With lower values the overshoot is too
high; with much higher - the front gets a "step".
The driver also seems to be nice - the rise/fall time is not too fast - 2.5ns typical,
and can drive up to 10 TTL loads.

Mike.

Juergen Hannappel wrote:

> Michael Nudelman <mnudelman@tellium.com> writes:
>
> > Juergen,
> >
> > How many stubs do you have on any given LVDS line on your backplane? If it is not
> > point-to-point, in the first place equalizing LVDS diff pairs on the board would be
> > a bad idea, since you want to keep stubs on your boards 1" or shorter. Again, only
> > if it is not point-to-point. Otherwise it's fine.
>
> The LVDS outputs are point-to-point. On the edge of the board i have a
> connector, and from there the signals go via twisted pair flat cables
> to other boards wich each have input termination.
>
> >
> > The 7ns difference would account for 42 (3.5 feet) inches, and you said your board
> > is roughly foot and a half. If the traces do not wiggle, and it is not some kind of
> > reflection you see - where the 7ns skew comes from? YOu should see no more than 3ns
> > skew worst case..
>
> I am also astonished by this, but the pattern of delays is the same
> for a number of boards (4), so it is not an erratic effect and also
> cannot be caused by the discriminator modules.
>
> >
> > The driver of 10125 can drive 50 Ohm line; if you have space problem - use 75/100
>
> Thanks for that info, i was unable to extract it from the data sheet...
>
> > Ohm lines (will make traces thinner and you could pack more of them), and since you
> > are point-to-point (TTL-to-LVDS input), use series termination; it will save you
> > instant drive current versus end-AC termination and power consumption versus
> > Thevenin split.
>
> It would not be point-to-point, but rather a 3 to 4 drop
> configuration, because i need 3 or 4 LVDS versions of the same TTL signal.
>
> >
> > Mike.
>
> --
> Dr. Juergen Hannappel Office: W148 Phone: +49 228 73 2447 FAX +49 228 73 7869
> mailto:hannappel@physik.uni-bonn.de Physikalisches Institut der Uni Bonn
> http://lisa2.physik.uni-bonn.de/~hannappe Nussallee 12, D-53115 Bonn, Germany
> CERN: Phone: +412276 76461 Fax: 77930 Bat. 892-R-A13 F-01631 CERN CEDEX, France

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