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I have a basic understanding on this topic but i still have some doubts, my text books says Differential means it will have impact on one of the 2 signal nodes. and Common will have a common impact on both nodes. There is no picture to show what nodes it is talking about! (If possible a description with some basic picture would be easy to understand).

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Imagine a perfect noiseless signal transmitted down two wires and you looked at what was received at the other end. Along the way, noise impregnates the two wires. What you might see at the receiving end is a noise voltage that exists between the two wires (differential). You will likely also see a noise voltage that affects both wires exactly the same with respect to ground (common mode).

The differential noise is effectively added to your signal and unfortunately, cannot be got rid of without special techniques. The common mode noise (both wires affected the same) can be eradicated by a decent differential amplifier i.e. it only amplifies the differential signal and is unaffected by common mode signals or noises.

Here's an example of a sudden spike of common mode noise affecting a differentially transmitted signal: -

enter image description here

The same noise signal affects both wires but, because a differential amplifier is used to receive the signal, that noise is cancelled out in the receiver.

However, if that noise affected one wire more than the other it would produce a differential noise and adds to the signal and cannot be eradicated easily. There are things that can be done but, I feel, it's beyond the scope of the question to go into these.

To avoid common-mode noises (the main source in a lot of installations) becoming differential noise, the following safeguards are observed: -

  • Differential signals are transmitted
  • Driving impedances (sending end) are matched
  • Differential amplifiers are used at the receiver
  • Receiving impedances to ground are matched
  • Decent twisted pair is used preferably with a screen

In addition to this, the cable at the receiving end may have a terminator to match impedances and prevent relections.

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  • \$\begingroup\$ Always the best answers from you !! So, a differential noise occurs when the signals on the two wires are affected differently by the noise! and common noise is when they are affected in the same way. \$\endgroup\$ – Hilton Khadka Apr 29 '16 at 10:34
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    \$\begingroup\$ Absolutely correct. \$\endgroup\$ – Andy aka Apr 29 '16 at 10:46
  • \$\begingroup\$ Also, is it correct to say that common mode noise are high frequency always? i was reading CM CHOKE and somewhere it says that it blocks common mode noise and somewhere it stops high frequency and allows low frequency to pass! \$\endgroup\$ – Hilton Khadka Apr 29 '16 at 13:33
  • \$\begingroup\$ NOise is generally not regarded as being high frequency or low frequency. Common mode chokes do deal better with high frequencies though. \$\endgroup\$ – Andy aka Apr 29 '16 at 14:13
  • \$\begingroup\$ ok!! so is it because the CM chokes are inductive and as a result it is less effective with low frequency noise but more effective with high frequency noise? \$\endgroup\$ – Hilton Khadka Apr 29 '16 at 14:17

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