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

  • From: "Beal, Weston" <Weston_Beal@xxxxxxxxxx>
  • To: "Santos Fernandez, Jesus" <jsantosfernandez@xxxxxxxx>
  • Date: Fri, 5 Dec 2008 08:35:26 -0800

Jesús,

It actually avoids confusion if we understand and use the proper terminology 
when discussing differential pairs. The modes are an intrisic property of the 
conductor geometry. They are specific states of excitation in the two 
conductors. This says nothing, yet about what signal is actual sent down the 
pair at any one time. The terms differential and common refer to the signals 
that are applied to the structure. There really is no such thing as 
differential mode and common mode. There is differential signal and common 
signal. We can also measure or determine the differential and common impedance 
of the differential transmission line by applying a differential or common 
signal to the structure. The differential impedance is that impedance that a 
purely differential signal sees as it propagates down the transmission line 
pair. Odd-mode impedance is the impedance of one conductor when the pair is 
excited in the odd mode. That's why Zdiff = 2*Zodd. The same applies to common 
signal, common impedance, and even-mode impedance; except that Zcommon = 
1/2*Zeven.

Since we send differential signals - and they always contain some amout of 
common signal in our non-ideal world - we want to terminate the differential 
impedance and the common impedance.

Notice that I did not use the terms common mode (CM) nor differential mode (DM) 
in this explanation. Those are the terms that get us confused when we try to 
figure out common impedance and differential impedance.

If you would like a more detailed explanation I suggest the book, Signal 
Integrity - Simplified by Eric Bogatin.

Regards,
Weston


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Santos Fernandez, Jesus
Sent: Thursday, December 04, 2008 11:50 PM
To: steve weir; King Da
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: [Help] How to do termination in differential signal with 
series termination ?

Is there any necessity to use the names of ODD and EVEN instead of the more 
descriptive CM and DM?

Jesús

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