Below are straight and twisted pair transmission lines:
From the above illustration the induced noise current due to the magnetic coupling are opposite direction in each lines.
It seems like magnetic coupling causes differential noise rather than common mode noise. Because the currents above are not flowing the same direction.
On the other hand, I read many places that STP wiring minimizes the induced noise and differential ended inputs are used to subtract the same noise on both lines.
But this illustration contradicts that. For diff-input amplifier to cancel out equal noises the currents due to the magnetic coupling must flow in same direction.
Where am I thinking wrong here?
In below diagrams there are two independent scenarios where the only source of noise around is assumed to be magnetic field such as 60/50Hz or so. In both scenarios the transmission line is perfectly balanced and twisted pair is used.
The only difference is that in Scenario I single-ended inputs used; and in the Scenario II differential-inputs used.
In this case, would the noise due to magnetic coupling be differential? And would using single ended or diff-ended inputs will not make any difference?