# A question about oscilloscope and common mode noise

To ask this question I needed to simulate two circuits as an example. Imagine we want to measure the voltage across a device output. Call the differential voltage across its terminals Vd. And imagine we couple a usual scope to its terminals to observe the voltage. Call AGND is the scope's own ground. And call the usual triangle reference symbol as the earth ground for this example.

Now on the left side circuit, if the scope is earth grounded and if there is a common mode interference as Vcm the simulation shows the green plot as the difference between Scope and AGND nodes. But if I cut the earth connection of the scope as on the right circuit the simulation shows the blue plot as the difference between Scope1 and AGND1 nodes.

In the simulation above, cutting the earth is hiding the common mode interference information. Is that what happens in reality? Can this be a way to check whether the noise is common mode or differential in nature?

Is that what happens in reality?

Try this instead (more like reality): -

You might want to try this at a higher frequency to really get a feel for high frequency interference too.

Note - I'm unsure why you are using 10 milli ohm resistors.

• 10mOhm just a wire resistance I made up there is no other reason. I tried your suggestion with 100MegHz 1pF and 220pF and when I measure V(Scope1,AGND1) it is extremely small comparing to the other earth grounded scope circuit. So it still hides CM information. All the reason Im doing this is that it has been long time I still don't know what is the practical way to tell whether what I see as noise on scope is due to common mode interference or differential mode. Im trying to find a way to tell it is CM or DM noise. – atmnt Dec 11 '18 at 14:06
• I mention the resistance because a proper scope probe will have maybe 1 Mohm to 10 Mohm resistance in series.like this – Andy aka Dec 11 '18 at 14:10
• Here is my challenge electronics.stackexchange.com/questions/411724/… – atmnt Dec 11 '18 at 17:45

You will see the common mode noise if your scope has even the most passing connection to earth ground. By connecting the scope ground to AGND1, you're overriding that (presumably) high impedance to earth ground, forcing the scope to use AGND1 as a reference.

If your scope's earth ground is low impedance, your'e just shorting Vcm.