# Why is noise is considered to be a "Common Mode Signal"?

I learnt that the magnetic interference due to the Earth causes noise in the cable but I wonder why this is considered to be a 'Common Mode Signal' and why differential amplifiers are used to eliminate it?

• The earth's magnetic field is static so can induce no noise. Solar flares can cause magnetic interference and these can induce voltages in cables but if signalling is differential and links are two wire differential impedance matched, then the induced voltage is the same in both wires and is common mode hence removed with a differential amplifier. Aug 25, 2017 at 15:03
• @Andyaka The earth magnetic field is static, but the system might be moving and vibrating, so theoretically the noise can be induced by Earth magnetic field. Aug 25, 2017 at 15:07
• Please go through this as well electronics.stackexchange.com/questions/79752/… Aug 25, 2017 at 15:33
• "can two wires have noises in opposite phases" the common-ness can vary based on the geometry of the wires and source direction and frequency of the noise. That's why we use twisted pairs, so that those effects are nullified to a great extent. Aug 25, 2017 at 15:36
• lol @Andyaka I was thinking the same thing when I wrote my comment... great minds ;) Aug 25, 2017 at 16:56

Firstly, this picture below hopefully explains why a differential signalling receiver will cancel out interference (or noise) on both wires: -

It's a simple case of A minus B i.e. the noise/interference gets cancelled but the wanted signal gets left intact.

Secondly, when an interference source is some distance away from the two-wire cable, it largely inflicts noise equally onto both conductors hence the noise is called "common-mode". When a noise source is much closer to one wire than the other there will be a noticibly differential noise signal and this isn't so easily dealt with by a receiving differential amplifier.

So, you have common-mode noise and differential noise and to make the incident noise only (mainly) have common-mode effects you need to do several of the things below: -

• Use matched impedances to ground so that any influence from noise sees equal impedances to ground thus, one wire does not naturally receive a larger noise signal than the other.
• Keep the noise source as far from the cable as possible
• Use differentail signalling to improve signal level amplitude (reduce SNR)
• Use twisted pair so that magnetic noise induces the same voltage on both wires. This also helps a bit with electric field interference.
• Use a screen so that electric fields couple to the screen and, due to internal capacitances, couple equally to both wires.
• Use a receiver that can deal linearly with signal and superimposed noise (transformers are good for this).

Noise will affect both the lines in similar way hence by using a differential amplifier you eliminate the signal which is common to both the input (which is noise).

Thats the reason you will find many op-amps witll have very high CMRR value which is common mode rejection ratio.

• You can learn more over here youtu.be/BFLZm4LbzQU Aug 25, 2017 at 15:18