Re: [SI-LIST] : Physcially-small far-end LVDS terminations?

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From: sweir (weirsp@a.crl.com)
Date: Tue Dec 21 1999 - 08:42:30 PST


Caveat emptor.

Regards,

Steve.
At 08:04 AM 12/21/99 -0800, you wrote:
>Jon,
>
>That's exactly why the 100 ohm resistor is needed. It's not a
>differential impedance matching device, it is intended to be two 50 ohm
>resistors terminating two 50 ohm lines to Vref. When this is done, the
>net current flowing in the Vref pin is zero, so the connection is omitted.
>
>100 ohm differential impedance is not needed. Make each line a good 50
>ohm line.
>
>Lee
>
>Jon Powell wrote:
>
> > Though I could easily be wrong, I don't believe that the below is true.
> My understanding of LVDS is that it requires ~100 ohm termination across
> the differential receiver in order to generate the required signal voltage.
> >
> > regards,
> > jon
> >
> > Jim Freeman wrote:
> >
> > > Hi Eric,
> > > Another answer would be to ensure that the differential impedance
> of the board between the differential pairs is 100 ohms. Then you
> wouldn't need the resistors. I have performed Hspice sims on this and it
> works fine if the stackup/trace spacing is correct.
> > >
> > > Jim Freeman
> > >
> > > Eric Goodill wrote:
> > >
> > > > Hi,
> > > >
> > > > I want to use the typical LVDS termination where you have two
> 50-ohm resistors in series placed between the two lines of the LVDS diff.
> pair and also have a cap at the junction of the resistors to ground. I
> have a jillion LVDS pairs to terminate at a high-pin-count ASIC, and I'm
> looking to squeeze more terminations per square inch. (There's no hope
> of putting the terminations inside the ASIC at this point.) I'm hoping
> some company makes a single three-pin part the has those two resistors
> and a cap even if not specifically for termination use.
> > > >
> > > > How about any other ideas for even higher density?
> > > >
> > > > Regards, Eric
> > > > --
> > > > Eric Goodill
> > > > Siara Systems, ericg@siara.com
> > > >
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> > --
> > Jon Powell
> > Director of HSSD Consulting Services
> > Viewlogic Systems, INC.
> > 805 988 8250
> >
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