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

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From: Ron Miller (rmiller@Brocade.COM)
Date: Tue Dec 21 1999 - 11:47:15 PST


If you terminate the common mode the common mode current will flow. If you can avoid
this you will have less problems with noise. After, all, what you are really
trying to receive is the differential signal. That is why you made it differential in the first
place, to isolate it from common mode currents and voltagees.

So, terminate in 100 ohms differentially, ignore the common mode voltage and isolate the current. No Vref is needed.

Also, if there is line to line skew, (time) the differential termination does not develop
the two distorted signals that occur with seperate terminations.

Ron Miller

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, ericg@siara.com
> > > > >
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> > > --
> > > Jon Powell
> > > Director of HSSD Consulting Services
> > > Viewlogic Systems, INC.
> > > 805 988 8250
> > >
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> --
> Eric Goodill Siara Systems
> ericg@siara.com 1195 Borregas Ave
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>
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--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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