# Ground offset or common mode noise voltage

This is just a small doubt I would like to clear.

Suppose I have a buck regulator providing output at 5V and 1A.

Suppose I measure the output voltage at the ground pad of the output capacitor and it measures like 0.2V.

Is this 0.2V common mode noise or is it ground bounce or what is the 0.2V? The neutral (return path) is not at 0V potential but at 0.2V. So, can I call this as common mode noise voltage or ground offset/ground bounce?

• Please be more specific where you measure. You can not measure the voltage at one point. you always needs two points. By definition if the two points collapse into one you have zero voltage. If not, noise is entering your measurement some way. Dec 17, 2019 at 3:50
• Sorry. One end of the probe is connected to the scope ground which is connected to the main board ground. Other end is probed at the capacitor ground pad
– user220456
Dec 17, 2019 at 4:08
• Ah. So this an oscope? Think about how big your path is from your PCB ground to the ground one the oscope. It could be creating a rather large antenna and you could be picking up some noise somewhere in the air.
– user103380
Dec 17, 2019 at 4:11
• You will always have some difference in voltage if there are currents flowing in your ground system. As this is a supply you should always measure at the output, not between arbitrary ground points Having said that: a scope is notorious for picking up noise if you have a 'loop' between your probe and your ground. Dec 17, 2019 at 4:13
• Is that 0.2V DC, or an AC waveform? Please show a photo of the main board and regulator so we can see the layout. Dec 17, 2019 at 4:47

We use the term "Ground" for a local voltage we choose to define as 0V. There does not even have to be any current flowing thru that connection. It is just a 0V potential reference.

Since it is a local reference, the term "ground" can be many used for different reasons.

The "Protective Earth" (PE) Ground is used for AC grid reference defined by some low resistance path to conductive earth to a distribution transformer Neutral tap below into conductive soil and to copper plumbing below earth surface level.

"Floating ground" just means voltage is high impedance to protective earth ground.

There can be many reasons for one local ground to differ from another.

Your example of 0.2V offset with respect to the scope's "earth grounded" measurement may be called an offset or a ground shift due to the current *resistance=voltage between the two 0V references.

It could be 1A * 0.2 Ohms of resistance or 1uA*0.2 MOhms, as your measurement does not include resistance or shared current which is required to tell the reason for the difference.