Re: [SI-LIST] : Combined single-ended/differential termination package?

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From: Eric Goodill (ericg@redback.com)
Date: Mon Feb 26 2001 - 21:17:53 PST


We generally use the two-resistor and one-capactitor termination shown
in my original question for LVDS. Seems to work well. However, I've
only ever run it up to 622 Mbps.

-Eric

Larry Miller wrote:
>
> ---which brings up something that has been bothering me:
>
> the LVDS standard goes with the single 100-ohm line-to-line resistor (which
> is REQUIRED for DC biasing) but does not seem to provide any specific line
> to ground termination. I know that LVDS was originally intended for cables,
> but it is showing up in a lot of 2-3 Gb signals, including serial
> backplanes.
>
> It looks to me as though you end up with unterminated (to ground) wires in
> the pairs.
>
> What do you do with this?
>
> Seriously.
>
> Larry Miller
>
> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Eric Goodill
> Sent: Monday, February 26, 2001 6:09 PM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Combined single-ended/differential termination
> package?
>
> Ron,
>
> Our reason for both resistors is to terminate any common-mode signal
> that might be on the lines either from crosstalk or from an unbalanced
> driver.
>
> -Eric
>
> Ron Miller wrote:
> >
> > Hi Eric
> >
> > Why not just use a single resistor to terminate the differential mode,
> > which is the signal you are intrested in.
> >
> > Tieing the center-tap to ground allows common mode currents to flow
> > which usually end up allowing power supply ground currents to get into
> > the sensitive signal paths.
> >
> > -----Original Message-----
> > From: Eric Goodill [mailto:ericg@redback.com]
> > Sent: Monday, February 26, 2001 3:47 PM
> > Subject: [SI-LIST] : Combined single-ended/differential termination
> > package?
> >
> > Hi,
> >
> > We often use this termination scheme:
> >
> > SIG+ -------+
> > |
> > R
> > |
> > +--C--+
> > | |
> > R V
> > | GND
> > SIG- -------+
> >
> > That is, two resistors in series between the differential signals with a
> > center-tapped capacitor to ground. Is there a combined device to sucks
> > all these parts into a three-pin package?
> >
> > Regards, Eric

-- 
Eric Goodill                    Redback Networks
ericg@redback.com               300 Holger Way
voice: (408) 571-5319           San Jose, CA 95134-1362

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