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

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From: Milt Schwartz x3261 (schwartz@galaxy.nsc.com)
Date: Mon Dec 20 1999 - 12:34:07 PST


I agree Jon.
The LVDS devices require a termination resistor. If compliant with TIA/EIA 644, then the resistor value that will be supported by the specified output voltage swing is 90 to 120 ohms (if I remember the spec correctly).

> From owner-si-list@silab.eng.sun.com Sat Dec 18 12:48:42 1999
> Date: Sat, 18 Dec 1999 09:08:31 -0800
> From: Jon Powell <jonp@pacbell.net>
> Subject: Re: [SI-LIST] : Physcially-small far-end LVDS terminations?
> To: si-list@silab.eng.sun.com
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> 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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