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

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From: Lee Ritchey ([email protected])
Date: Thu Dec 23 1999 - 09:20:59 PST

Correct. The single 100 ohm resistor assumes there is little common mode. If this is taken into account, three resistors are needed. One across the pair of lines and two to "ground". If the modelling is done showing both types of impedance, ignoring the common mode signal is acceptable in almost all cases. Remember, LVDS is tolerant of substantial mismatches.

We are talking about making the tradeoff between perfect and good enough. It's and economic one as well as a performance one. The whole business of making good high speed products that work and are economical turns around this concept.

Lee

Eric Goodill wrote:

> Lee Ritchey 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
>
> Lee,
>
> In general, doesn't one need to terminate both the differential and the
> common modes? The 100-ohm-only termination terminates the differential
> mode but not the common mode. I think in this case we're just relying
> on the fact that there's very little common-mode signal, so it doesn't
> get in the way.
>
> -Eric
>
> > 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, [email protected]
> > > > >
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> > > --
> > > Jon Powell
> > > Director of HSSD Consulting Services
> > > Viewlogic Systems, INC.
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> --
> Eric Goodill Siara Systems
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>
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