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

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From: Marc Humphreys (mhumphreys@nexabit.com)
Date: Fri Dec 17 1999 - 14:36:45 PST


My simulation show something quite different.
The 100 ohm far end termination between the two differntial input
is required at the far end of the line to generate the differential
voltage at the receiver. The LVDS drivers are typically 2 to 4ma
current sources, which subsequently generate the
200 to 400 mv voltage swings at the receiver when the current flows
out one side through the resistor and back thru the other side. No
terminator
and my _P and _N signals stay pretty static at ~ 0.9v and 1.5vrespectively.

Jim, maybe the spice model for your using receiver has a built in 100 ohm
terminator.

There are a number of ways to generate the required input
voltages at the input of an LVDS receiver, hence the reason for
asking the vendor, is this a true LVDS implementation. God knows there all
kinds of ways of taking ECL, PECL and CML etc. signals and massaging them
to meet the LVDS specs but its a far cry from the simplicity of a low
current source
single, a 100ohm diff pair set of traces, and a single far-end terminator,
discrete or
on chip terminator.

My 2 cents.

Marc

My 2 cents.

Marc
> -----Original Message-----
> From: Jim Freeman [SMTP:freeman@broadcom.com]
> Sent: Thursday, December 16, 1999 4:33 PM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Physcially-small far-end LVDS terminations?
>
> 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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