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Differential pair common mode return path

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neoflash

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differential pair without return path

Hey, friends:

Assume there are only two metal traces in the free space, and no other conductor.
Since there is no common mode return path, the common mode resistance is infinity, is that right?

Can I say that there is no common mode current flows in the diff-pair?

thanks,
 

Hi, Han: You can say that if it is far away from any discontinuities. Basically, the common mode is not a propagating mode. Any common mode energy will be reflected or radiated away from any discontinuities. When you have a long differential pair without the 3rd metal around it, you will get pure differential signal at the other end no matter whether the stimulus at the input is differential or not.
 

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    neoflash

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thanks for the answer.
Well, let us assume that we have a open-drain driver drives the diff-pair from the middle of the TL.

I will terminate at both far ends.
If the common mode impedance is infinity. Can I say that the common mode voltage I see at the driven node is infinity? (since Vcm=Gm*Zcm)

That could be very terrible!
 

Hi, Neoflash:

You will not get infinite V. Basically, when you drive a differential pair at the the middle of a TLN, you are actually creating some discontinuties into the structure. You will have common mode around it. The Zin at the feed point will not go to infinity while it will be a big value. Therefore, if you try to inject some current into it, you will be able to inject some slight current into it while it is very difficult to build it up. I think the situation is simialr to you have a tank of water and its cover is sealed very well. Then, you try to push the cover down. Practially, you will be able to push down very slightly and the pressure you put on it will not go to infinity. Howver, it will be very difficult for you to push it down significantly. It is basically the same situation.

Best regards,
 

What about my driver have infinite large Zs?

Assume it is a open-drain driver?

I will do some experiment in hspice.
 

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