# RE: [SI-LIST] : Differential impedance

Dima Smolyansky ([email protected])
Wed, 11 Nov 1998 21:45:19 -0800

The point is that in digital design "differential" is identical to "odd =
mode." In other words, in digital system the signals on a differential =
line will have different polarities, thus only an odd mode propagates. =
Therefore the formula Z =3D 2*Zodd should work quite fine for all =
practical purposes.

Thanks,

-Dima
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D
Dima Smolyansky
Marketing and System Development
TDA Systems, Inc.
7465 SW Elmwood St.
Portland, OR 97223
(503) 977-3629
(503) 245-5684
[email protected]
http://www.tdasystems.com

The Interconnect Modeling Company(tm)

-----Original Message-----
From: fabrizio zanella =
[SMTP:fabrizio=3Dzanella%eng%[email protected]]
Sent: Wednesday, November 11, 1998 4:04 PM
To: [email protected]
Subject: [SI-LIST] : Differential impedance

To calculate the differential impedance of a transmission line pair, =
I've=20
always used=20
1) Zdiff =3D 2 * Zoo, where Zoo is the odd mode impedance of the pair. =
This=20
formula is used by the pc board manufacturers.
I was speaking to someone last week who has 30+ years in the industry =
and=20
said this formula is wrong, that the real differential impedance should =
be=20
calculated as
2) Zdiff =3D square root of (Zoo * Zee), where Zee is the even mode=20
impedance. =20

If I compare 1) to 2), I get quite different values of differential=20
impedance. Does anyone have comments on this? =20

Thanks, Fabrizio Zanella
EMC corporation
[email protected]

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