# Differential Probe Ratings

KyleGranger
TL;DR Summary
I am trying to understand different ratings on a differential probe
Hi,

I am looking at a Micsig DP2003 high voltage differential probe. It's rated for 5.6 kV differential voltage. I understand that the differential votlage is measuring across two terminals which can be floating (not referenced to GND). The specs say it is rated for 1 kV common mode voltage, but I don't understand that. I thought the common mode voltage was difference in voltage between one terminal being measured and GND. I don't think my understand of this is right because if so, how would you ever measure >1 kV with your scope referenced to GND? If this is correct, does that mean I would have to float my scope to truly measure >1 kV differential?

Assuming my understand is incorrect, how do I determine the maximum voltage that I can measure? For instance, let's say that I have 10 capacitors in series with 2 kV across each. I would have a 20 kV bus. If I wanted to measure the voltage across the top capacitor, it would only have a differential voltage of 2 kV so would be fine by the DM voltage spec. But wouldn't the CM voltage be 20 kV and blow up? What's the highest cap that I can measure in this 10 capacitor string using this specific probe? A link to this probe with specs is below.

https://www.amazon.com/dp/B07DKT61LD/?tag=pfamazon01-20

Assuming my understand is incorrect, how do I determine the maximum voltage that I can measure?
“Max differential test voltage (DC+AC PK-PK): 560V (200X); 5600V (2000X)”
“Max input common mode voltage: 1000V CATIII”
That suggests to me the specifications have been confused.

The differential voltage is the voltage between the two probes.
But that is also the common mode voltage, specified as 1000 V max.
So you will not be able to measure 2 kV across a capacitor.

So the 560V or 5600 V must be the voltage on either probe relative to the ground.
Maybe you need to go to the manufacturers data sheet to get the correct details.
http://micsig.com/html/41.html

KyleGranger
“Max differential test voltage (DC+AC PK-PK): 560V (200X); 5600V (2000X)”
“Max input common mode voltage: 1000V CATIII”
That suggests to me the specifications have been confused.

The differential voltage is the voltage between the two probes.
But that is also the common mode voltage, specified as 1000 V max.
So you will not be able to measure 2 kV across a capacitor.

So the 560V or 5600 V must be the voltage on either probe relative to the ground.
Maybe you need to go to the manufacturers data sheet to get the correct details.
http://micsig.com/html/41.html
The unfortunate thing is their user manual says the exact same thing. What I am looking for is the isolation voltage I think. I've used these probes to measure about 1.25 kV differential and about 10 kV common mode (with respect to ground). It just hit me that I should check what the actual CM rating is before I push it any higher. I won't bash their manual here but it looks like I'll just have to find a different probe that lists the proper ratings. I was really hoping it was just me not understanding what I was reading.

Gold Member
Sorry, I didn't actually look at your device data. The thing to keep in mind with common mode voltages is that they are referenced to a common ground, which is normally the Earth ground of your oscilloscope or equivalent. So for two voltages V1 and V2 referred to ground, the common mode is Vcm = (V2+V1)/2. The difference mode is normally Vdm = (V2-V1)/2, but some assume it is just the difference V2-V1.

KyleGranger
Sorry, I didn't actually look at your device data. The thing to keep in mind with common mode voltages is that they are referenced to a common ground, which is normally the Earth ground of your oscilloscope or equivalent. So for two voltages V1 and V2 referred to ground, the common mode is Vcm = (V2+V1)/2. The difference mode is normally Vdm = (V2-V1)/2, but some assume it is just the difference V2-V1.
So if I understand you correctly, using the example of 10 series capacitors at 2 kV each for a 20 kV bus...if I were to measure the top capacitor one terminal would be 18 kV WRT GND. Each capacitor has a differential voltage of 2 kV so the 2nd terminal would be 20 kV WRT GND. You're saying the differential voltage, relating to ratings, would be only (20kV-18kV)/2= 1 kV?

As far a CM voltage, it would be (20kV+18kV)/2=18 kV?

That's all news to me because I would expect the DM voltage to be 2 kV and CM voltage to be 20 kV.

Gold Member
So if I understand you correctly, using the example of 10 series capacitors at 2 kV each for a 20 kV bus...if I were to measure the top capacitor one terminal would be 18 kV WRT GND. Each capacitor has a differential voltage of 2 kV so the 2nd terminal would be 20 kV WRT GND. You're saying the differential voltage, relating to ratings, would be only (20kV-18kV)/2= 1 kV?

As far a CM voltage, it would be (20kV+18kV)/2=18 kV?

That's all news to me because I would expect the DM voltage to be 2 kV and CM voltage to be 20 kV.
The CM voltage is the average voltage wrt ground Vcm = (20kV+18kV)/2=19 kV.

The differential voltage (according to this definition) is Vdm = (20kV-18kV)/2= 1 kV, the voltage away from the common mode voltage.

The voltage difference is (20kV-18kV)= 2 kV.

Note that each individual voltages can be reconstructed from (Vcm + Vdm) and (Vcm - Vdm). So it's a nice linear decomposition to a different basis.

But these are just definitions that turn out to be useful in analysis (noise & differential amplifiers, mostly). So, make your own definitions if you like. But be aware that experienced EEs think you mean their "normal" version.