How is the differential mode noise suppressed by adding a capacitor across the Lines

Question

I was reading about the common mode noise and differential mode noise.

I don't understand how a differential mode noise gets coupled and how adding a capacitor across the line will help.

I understand about common mode noise. Like, if we have a noisy source, it might equally couple on both lines and we can use a common mode choke to reflect back the common mode noise back to the upstream source.

But how is a differential noise generated on the lines? Is it generated by places the lines close to differential signals?

My questions:

1. In general, does the noise affect only power lines or signal lines as well? I am asking because, I have seen app notes and other illustration depicting these concepts only in the context of Power supplies.

2. Is differential mode noise, generated on the lines, by placing the differential mode signals near them? Or how else, will the line and neutral wires have noise voltage of opposite polarity? I assume that differential mode noise is obtained only by placing the circuit near differential mode signals. Am I correct in this understanding?

3. And I understand how a common mode choke attenuates the common mode signal. But how does adding a capacitor between the lines, attenuate the differential mode noise? Can someone provide some help on this.

Thank you

Answer 1

Most interfering noise on a cable is coupled equally to both conductors when certain cabling precautions are taken and these are twisting pairs (to reduce magnetically induced effects) and screening (to reduce electrically coupled effects).

But, to ensure you handle correctly the common mode voltages on both conductors in a pair, you need to have a balanced differential receiver.

The word “balanced” is key here. The balancing comes from both inputs having the same impedance to ground. If the impedances were not equal, most of the effort of screening and twisting is wasted.

Does a balanced receiver have perfect earth impedance balance? No, and that’s how a common mode interfering signal can result in a differential interference despite making great efforts with the cable.

However, any differential interference is usually high frequency and, this can be reduced by a differentially placed capacitor.

Also remember that an Earth impedance imbalance can be created at the transmit end so, if you don’t have an impedance balanced driver, all the great efforts in choosing the best cable and designing a decent receiver are spoilt by the transmitter’s poor characteristics.