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

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From: Larry Miller (ldmiller@rhapsodynetworks.com)
Date: Mon Feb 26 2001 - 20:10:31 PST


---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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