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Lets say I have 2 ICs- C1 and C2 acting as driver and receiver for the digital signal. The signal is transmitted through a conductor/cable ( 2 wires) as shown below. I have 2 doubts

1. What would be the direction of current flow through GND wire? Will it be alternating? From my limited knowledge in semiconductors, I think when the signal is high, it would charge the internal capacitor of receiver and discharge the same cap when the signal is low. Hence I expect an alternating ( +ve and -ve) current through the cable. Is it correct?

2. Now why would the return current choose the alternate return path? The current would choose the least impedance path, and for an alternating current it is obviously the gnd return path through cable (least inductance path). If any dc component is there it may divide and flow through alternate return path as well.

3. Now what is the problem if at all some dc current flows through alternate return path? There is no EMI issues caused by a dc current.

  1. Now lets say a small fraction of ac current flows through earth return path. Is it correct to consider it as a current loop since both the return currents are flowing in the same direction and not really circulating?

So much is written on this which I need to research more . I will proceed after getting clarity on atleast 1st two questions enter image description here

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  • \$\begingroup\$ Forgot Kirchoffs current law ... ? \$\endgroup\$
    – Mitu Raj
    Oct 23, 2019 at 3:59
  • \$\begingroup\$ So are you saying, a small fraction of alternating current flows through earth ground return path which causes all the problems? \$\endgroup\$
    – Divya K.S
    Oct 23, 2019 at 4:19
  • \$\begingroup\$ @MituRaj Edited the question after rethinking \$\endgroup\$
    – Divya K.S
    Oct 23, 2019 at 4:27

2 Answers 2

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Return current will be mostly in the shield of your coaxial cable, but that's not the problem.

The problem is any changing magnetic field through that ground loop (the co-ax shield and the two ground connections) will induce a current that flows in the co-ax shield and that current will cause a voltage that looks like a signal to the receiver

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  • \$\begingroup\$ Thanks for the crisp and concise reply \$\endgroup\$
    – Divya K.S
    Oct 23, 2019 at 9:28
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    \$\begingroup\$ Current is not induced, voltage is induced. \$\endgroup\$
    – Andy aka
    Oct 23, 2019 at 9:57
  • \$\begingroup\$ on what basis do you say that current is not induced? what if I drop a magnet through a copper pipe, or a superconducting tube? \$\endgroup\$
    – Jasen
    Oct 26, 2019 at 6:48
  • \$\begingroup\$ A changing magnetic field induces voltage (electromotive force), which causes current to flow if there is a path for it. In a part of a circuit (eg. coax shield) we could say that the current going through that part produces a voltage across its resistance, but somewhere a voltage must be producing the current. Induced Current from Changing Magnetic Field \$\endgroup\$
    – Bruce Abbott
    Oct 26, 2019 at 7:49
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Electrons explore ALL possible paths and take ALL paths, proportional to 1/impedance. Did I not clearly write ALL possible paths, including thru the air as displacement-currents, and thru that 6 meter longer path around the framework of the card-cage. ALL possible paths.

Why is this? because this exploration, and exploitation, of ALL possible paths is how nature minimizes the energy of that part of the system.

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