l impedance vs common mode impedance

[Slav]

Dear community,
can someone explain to me some misunderstandings. There is a differencial
pair in a PCB. Simulation returns: Impedance 65.5 ohm, differencial
impedance 98 ohm, common mode impedance 82 ohm..I wonder what is the
difference between 'impedance' and 'common mode impedance'? Just 'impedance
is (as I see it) the impedance of a trace, if that trace was alone.Well.But
what common mode impedance is? Any links or tips would be appreciated.
Slav.

 

[Georg Baum]

Slav wrote:

Dear community,
can someone explain to me some misunderstandings. There is a differencial
pair in a PCB. Simulation returns: Impedance 65.5 ohm, differencial
impedance 98 ohm, common mode impedance 82 ohm..I wonder what is the
difference between 'impedance' and 'common mode impedance'? Just
'impedance is (as I see it) the impedance of a trace, if that trace was
alone.Well.But what common mode impedance is? Any links or tips would be
appreciated. Slav.

This should be explained in the manual of your simulator. In case it is not,
ask google: Searching for "common mode impedance" gives as hit no. 12
http://www.ultracad.com/articles/diff_z.pdf, which should get you started.


Georg

 

[Joel Kolstad]

Slav wrote:
can someone explain to me some misunderstandings. There is a differencial
pair in a PCB. Simulation returns: Impedance 65.5 ohm, differencial
impedance 98 ohm, common mode impedance 82 ohm..I wonder what is the
difference between 'impedance' and 'common mode impedance'?

"Impedance" should be the characteristic impedance of one trace of the
differential pair if it were used in isolation somewhere as, e.g., a
microstrip line. "Differential impedance" is the impedance seen by a fully
differential signal traveling down the pair (e.g., 1V on one trace, -1V on the
other). "Common mode impedance" is that seen by a common mode signal
traveling down the pair (e.g., 1V on both traces... or -1V on both traces).

Differential impedance is also known as "odd mode" impedance, common mode
impedance is also known as "even model" impedance.

Well.But what common mode impedance is? Any links or tips would be
appreciated. Slav.

The link Georg gave is a good one. In general, differential impedance will be
something LESS than TWICE the "regular" impedance because the coupling between
the traces "help" one another (since the currents in each one travels in
opposite directions -- from a lumped circuit perspective, the mutual
inductance of the two traces is negative) -- they'd really like to get closer
together if they could! The twice comes about since each traces has its own
terminating resistor that ends up in series between +/-1V. Likewise, common
mode impedance will be something MORE than HALF the regular impedance, where
the "half" comes from the terminations appearing in parallel and the "more"
coming from the two traces "fighting" each other since they're now carrying
now the same current (mutual inductance is positive) and they'd prefer to get
away from one another if possible.

In your case, we can compute the coupling coefficient between the traces as
0.252 (see the link for the math), so your odd mode impedance should be about
41 ohms. For some reason, you tool is giving you twice that -- 82 ohms; this
is probably just a convention your particular tool uses.

Any old signal across two traces can be viewed as a sum of a common mode and
differential mode signal, where Vcm = (V1+V2)/2 and Vdm = (V1-V2)/2.

If you know that your system is using differential signalling all the time,
you'd termiante your differential pair in the differential impedance your tool
is giving you. However, if that "differntial pair" sometimes carries common
mode signals are well (USB, for instance, does this), there's no particularly
simple way to properly termiante the 'pair for both modes, so sometimes people
will terminate in the geometric mean of the even and odd mode impedances as a
starting point (63.4 ohms in your case) and then take a look at the results.
(Although, for USB, 99% of the time the signals really are differential, so
they terminate much close to the differential impedance...)

A coupling coefficient of 0.252 is actually pretty high for most applications,
so you probably will have to experiment a little to find the best termination
possible.

---Joel Kolstad

 

[Joel Kolstad]

I realized I made an error here...

"Joel Kolstad" <JKolstad71HatesSpam@yahoo.com> wrote in message
news:11ik7kmn9q2bt8c@corp.supernews.com...

so sometimes people will terminate in the geometric mean of the even and odd
mode impedances as a starting point (63.4 ohms in your case) and then take a
look at the results.

This should be sqrt(2)*63.4 = 89.6 ohms, since the common mode impedance needs
to be doubled to get the total series value of the terminators.