[SI-LIST] Re: LVDS center tap capacitor termination

Steve, 
       one general question, hope it will clear my
confusion. What is the even mode impedance of tightly
coupled diff pair (used in LVDS) which has diff
impedance 100 ohm ???
Thanks in advance,

Shekh 

--- steve weir <weirsp@xxxxxxxxxx> wrote: 
> No, that isn't right.  Take the view Lee likes to
> emphasize, and that is at 
> the driver, we have two single ended signals that
> are coded as complements 
> with low skew.  If we do not couple these lines
> significantly, and blast 
> them down a Tx line, then we need to terminate each
> line into the 
> single-ended impedance of 50 ohms.  But 50 ohms
> needs to be to a Vtt that 
> is at the center of the swing.  The capacitor does
> that.
> 
> Now, if we couple the lines tightly, then we would
> want to add a fourth 
> resistor in place of the original 100 ohms to match
> the line to line 
> coupling.  This would be something like 500 to 1000
> ohms.
> 
> Steve.
> At 12:04 PM 8/30/2004 +0100, shekhar sharma wrote:
> >Steve,
> >
> >Thanks for correction. Some mixing....
> >Anyway, I think, this termination terminates the
> even
> >mode (AC termination) upto some extent, coz the the
> >resistance values are 50 ohm each with their common
> >point connected to a capacitor. Diff pair with 100
> ohm
> >diff impedance will have common mode impedance more
> >than 50 ohm.
> >
> >
> >  --- steve weir <weirsp@xxxxxxxxxx> wrote:
> > > You are right that it terminates both modes. 
> You
> > > just have the modes
> > > confused.  Even is the common mode, odd is the
> diff.
> > > mode.  The AC
> > > termination is to the even mode.
> > >
> > > Steve.
> > > At 11:37 AM 8/30/2004 +0100, shekhar sharma
> wrote:
> > > >Two resistor with centre tap capacitor
> termination
> > > >provides AC termination to odd-mode impedance
> which
> > > >terminates common mode signal.
> > > >May I right ???
> > > >
> > > >Shekh
> > > >
> > > >
> > > >  --- steve weir <weirsp@xxxxxxxxxx> wrote:
> > > > > The two resistor termination is a double
> > > > > single-ended termination.  The
> > > > > single resistor termination presumes a
> virtual
> > > > > ground at the center of the
> > > > > resistor.
> > > > >
> > > > > What noise sources do you think LVDS is
> trying
> > > to
> > > > > reject?
> > > > > What noise sources do you think LVDS can
> reject?
> > > > > What noise sources do you think LVDS cannot
> > > reject?
> > > > >
> > > > > Steve.
> > > > > At 03:34 PM 8/30/2004 +0530, Ummalaneni,
> Venu
> > > Babu
> > > > > (Venu) wrote:
> > > > > >Hi=20
> > > > > >
> > > > > >This query is regarding termination schemes
> at
> > > the
> > > > > LVDS receiver inputs.
> > > > > >
> > > > > >Can some one explain me the difference
> between
> > > a
> > > > > 'single resistor
> > > > > >termination' and 'the two resistors with
> center
> > > tap
> > > > > capacitor' ?
> > > > > >
> > > > > >I read some where that the 'center tap
> > > capacitor
> > > > > scheme' filter's  the
> > > > > >common mode noise. My doubt is that even in
> > > 'single
> > > > > resistor scheme'
> > > > > >also if we run the differential traces
> verly
> > > > > closely the common mode
> > > > > >noise pick-up by the two traces is equal
> and
> > > the
> > > > > Receiver can reject
> > > > > >this common mode part, right? So why we
> need to
> > > go
> > > > > for center tap
> > > > > >capacitor scheme? It unnecessarily increase
> the
> > > > > component count on the
> > > > > >board, right?
> > > > > >
> > > > > >Any inputs are appreciated.
> > > > > >
> > > > > >Best regards
> > > > > >Venu Babu.
> > > > > >
> > > > >
> > > >
> > >
> >
>
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