How does placing cap between signal GND and earth GND solve common mode noise?

Question

Thanks in advance for your time.

I am reading several articles about differential mode noise and common mode noise. Among which I found the below 2 pretty instructive:

https://www.murata.com/~/media/webrenewal/products/emc/emifil/knowhow/26to30.ashx https://micro.rohm.com/en/techweb/knowledge/emc/s-emc/01-s-emc/6899

Let me first talk about my understanding as a start point.

1. Differential mode noise is some noise generated with the propagation of power line, such as noise generated from some on-board chips' operation like source/sink power to GND. This will produce ripple on power line, which is noise. In this case, placing capacitor between power and GND line solves the problem, it conduct the noise current to GND and flow back to power source. This is illustrated in Figure marked 1 and 2.

2. Common mode noise is pretty confusing. It is generated through the coupling between your on board signal trace or plane and the Earth GND. The start point is, there is coupling between your on board trace or plane and the surrounding mechanical box which is connected to Earth Ground, so this coupling will generate some current through it. Then this coupling current will flow back to your power supply's Earth GND, which its surrounding mechanical box is connected to, then through its stray capacitance, the noise will flow to both the power and GND lines that the power supply feed to your board. This is the signal flow of common mode noise.

Is my understanding correct?

My questions are:

• In the first article, Suppression method of common mode noise (2), it says adding cap between signal GND and earth GND solve common mode noise, why? I understand method (1), using common mode choke, but why method (2) also works? Just because common mode noise is referenced to Earth GND, so placing cap between where common mode noise flow through and Earth GND, can filter it out?

• Why doesn't differential mode noise's current go with the common mode noise's path?

Answer 1

The commom mode noise exists between both the power supply lines and the reference GND. Without any measures, the commom mode noise is visible at the load over the stray capacity to GND. If you put caps from the supply lines to reference GND, the circuit of the noise source is shortend before reaching the load.

Answer 2

Ok, well this takes me back a couple of decades when I was in a discussion with an audio electronics engineer on how the power noise analysis can be explain in theory format to the audio techs. No one really came out with a good way of explaining it, but I will give you my perspective on it. I copied this from a pro audio forum, but since it was about this, this help you out. The only difference here is that in pro audio we have circuits with dual supply, and signals not always referenced to ground, which are considered balanced signals the could be in transmission lines or through specially designed circuits. So:

From here.

You see, when a power supply rectifies the signal to DC, the noise is superimposed on the rectified amplitudes on both the + out and and - out in phase. Now if we have two supplies in the circuit (like a +15 and -15 rails) we have one supply with the - out tied to a "grounding point" for the positive supply and the other supply's + out is tied to the "grounding point" to make the negative voltage supply. now what has happened is at the "grounding point" the noise from both supplies are added.

Now lets look at the states of noise that has developed with this dual supply with common DC ground. Which are defined like this:

Positive Supply Unbalanced Common Mode Noise: This is the noise measured and observed when referencing the Positive supply to the common DC ground.

Negative Supply Unbalanced Common Mode Noise: This is the noise measured and observed when referencing the Negative supply to the common DC ground.

Differential Supply Balanced Common Mode Noise: This is the noise measured and observed when referencing from the Positive Supply to the Negative Supply. Ideally, we would like to see a total cancellation here, and the ideal setting of equal and opposite supply voltages cancel the most noise deferentially.

Now to apply decoupling a certain way for suppression of diode junction noise, rfi, ect from the power supply depends on what noise source you want to suppress. Like the noise difference that develops between two grounds or two internal ac signal returns.

But you have to take in consideration: Where you are referencing the signal. Because an ac signal transmission line doesn't have a any dc ground reference, and internally, the ac signal in a circuit has several signal returns. They are the dc power ground, the positive dc supply, and the negative dc supply, and the inversion of the signal itself.