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

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From: Chan, Michael (Michael.Chan@compaq.com)
Date: Tue Dec 21 1999 - 11:09:19 PST


I am sure one of the few who needs one for my system if I have a choice.

MC

-----Original Message-----
From: bishop@ICD.Teradyne.COM [mailto:bishop@ICD.Teradyne.COM]
Sent: Tuesday, December 21, 1999 12:55 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Physcially-small far-end LVDS terminations?

You say "for a reflection free system...". Who wants a reflection free
system? As long as there
are no differential reflections, and as long as all receivers on the line
reject common-mode,
what little common-mode signal there is can bounce back-and-forth all day
for all I care. Of
course for low EMI and cross-talk we'd like no common-mode at all, but
that's a different issue.

Charlie Bishop

>
> I strongly disagree. The differential impedance for the line is definitely
needed. Otherwise,
there will be reflections at the terminator/transmission line interface for
the differential mode of
the signal. The common mode will not be affected because the approach you
suggest will handle the
common mode. For a reflection free system, BOTH 50 ohm transmission lines
and 100 ohm differential
impedance for the transmission line is required along with a 100 ohm
tremination across the
differential pair.
>
> Jim Freeman
>
> 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
> >
> > 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
> > > > >
> > > > > **** To unsubscribe from si-list: send e-mail to
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> > >
> > > --
> > > Jon Powell
> > > Director of HSSD Consulting Services
> > > Viewlogic Systems, INC.
> > > 805 988 8250
> > >
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