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If there is no third conductor between source and load, is there common-mode noise?

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parachutes30

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for example, ac adapter provide dc power to the notebook only with positive and negative terminal and there is no third conductor between them. Negative conductor maybe tied to the earth in ac adapter, or usually not. Is there still common-mode noise?
 

I think this is more of a philosophical question than a practical one. Common-mode voltage is, by definition, voltage that is the same on both lines, referred to some reference like ground. If your negative terminal is tied to ground, then I suppose there is no common mode voltage. Why do you ask?
 

oh.. some practical question in my head.
1, a AC-line powered bench DC power supply connects to a load with positive and negative terminal. The load has a metal case which is floating now. someone tell me that there is still common-mode noise. I am wondering..
2, two independantly powered CAN node communicate well only with CANH and CANL connected to each other and there is no common ground between them, do we still need common mode choke at the CANH CANL input of each transceiver?

barry said:
I think this is more of a philosophical question than a practical one. Common-mode voltage is, by definition, voltage that is the same on both lines, referred to some reference like ground. If your negative terminal is tied to ground, then I suppose there is no common mode voltage. Why do you ask?
Click to expand...
 

1) There is still common-mode noise, as long as you're referencing a third point. If everything is TRULY floating (which I'm sure it isn't) then there would be no common-mode noise. It has to be "common" referenced to something.

2) There is no common ground between them, but there is common mode noise. Assume node "A" has its reference at 5000V, and node "B" has its reference at 0V. As far as node B is concerned, You've got 5000V of common-mode voltage. Not a good thing.
 
Even without additional external connections there's still ground capacitance and hence potential common mode noise.
 
Ground is ground until it isn't ground.

Like, both ends of a long wire can't possibly both
be ideal ground (unless you have an ideal plane
of zero L, R, C in your back pocket).

Your "ground" (return) wire is usually "signal" if
you look close (or care) enough.
 

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