# [SI-LIST] Re: [Help] How to do termination in differential signal with series termination ?

• From: olaney@xxxxxxxx
• To: yardala@xxxxxxxxx
• Date: Thu, 4 Dec 2008 22:33:24 -0800

```Series termination implies a round trip reflection, and that is exactly
what you don't want to do with a common  mode signal unless the intent is
termination at the receiving end to parallel terminate both modes, pi or
Y configuration depending on relative impedances.  You can use just two
resistors if Zcm = Zdiff/4, with each = Zcm/2 to ground.  Better yet,
suppress the CM signal at the sending end with a CM choke.  I presume
that any CM signal is unwanted, and not some sort of independent channel
as is sometimes used in telephony, or for PoE (power over ethernet).

Orin Laney

On Fri, 5 Dec 2008 14:20:09 +0800 "King Da" <yardala@xxxxxxxxx> writes:
> Dear Steve,
> "you've got
> your impedances backward" ? Could you explain more detail ?
> I use 100 Ohm for differential termination for diffrential signal
> and also
> want use series termination for each line for common mode signal
> termination. How to choose the series resistor value ?
>
>
> On 12/5/08, steve weir <weirsi@xxxxxxxxxx> wrote:
> >
> > Parallel terminations absorb energy at the far end and is related
> only to
> > the channel, not the driver Z.    Also, you've got your impedances
> backward.
> >
> > Steve.
> > King Da wrote:
> >
> >> Hi,Folks,
> >> I have a differential pair which odd mode impedance is 48 Ohm and
> even
> >> mode
> >> impedance is 108 Ohm by TDR measurement.
> >> I would like use 100 Ohm for differential termination at far end,
> but how
> >> to
> >> choose series termination resistor for each trace at near end?
> >>
> >> I refered Bogatin's "Signal Integrity:Simplified" book that said
> parallel
> >> termination R2 equal Zeven, So, I should use 108 Ohm subtract 20
> Ohm of my
> >> driver's Ro is 88 Ohm for my series termination at near end, am I
> right?
> >>
> >> The Rseries = 108 (Zeven) - 20 (Driver's Ro) = 88 Ohm, right ?
> >>
> >> Does the differential termination resistor is 100 Ohm better than
> 91Ohm ?
> >>
> >> Thanks,
> >>
> >> Yardala
> >>
> >>
> >>
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> >
> >
> > --
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