Hello.
I am using the following Keysight probe: https://www.keysight.com/gb/en/product/N2792A/differential-probe-10-1-200-mhz.html to probe a gate drive output voltage. The expected output is +15V and -5V. However when checking into the oscilloscope with 1Meg and 10:1 attenuation, the actual signal shows as +30V and -10V. The probe has a 10x attenuation and is not configurable. It also has a +\~20V peak differential voltage. I have tested the power supply of the gate drive and it is as expected without switching enabled. I have not yet tested the voltages while the gate drive is actively switching.

is there any commonly known explanation as to why a probe may show 2x the voltage magnitude when connected to a scope? If so is there any way to eliminate this issue?

 

Easy, check it out with a DC supply to see what you actually read on the probe.
If it still reads incorrectly, it could be a probe malfunction.

 

Easy, check it out with a DC supply to see what you read on the probe

Ah yes the simplest solution is always the best…

I’ll update the thread again tomorrow if I’m still reading double.

 

The 'scope you're using does have a 50Ω input impedance?

 

The 'scope you're using does have a 50Ω input impedance?

it does but I’m using the 1Meg input impedance. I checked this morning and the scope reads:

+36.8V
-11.6V

the multimeter is attached to the probe and reads:

+17.58V
-5.437V

the probe has an offset calibration mode which I attached to my 5V power supply but at minimum offset with the screwdriver it still reads that as about 8V. I tested again and the scope reads pretty much the same voltage as it did before the calibration.

 

it does but I’m using the 1Meg input impedance.

The probe expects 50Ω input. Using a 1MΩ input will give double voltage readings.

 

The probe expects 50Ω input. Using a 1MΩ input will give double voltage readings.

So I set the probe to 50R? I guess the input impedance of the probe is 1Meg from data sheet but it outputs 50R to the scope?

 

So I set the probe to 50R? I guess the input impedance of the probe is 1Meg from data sheet but it outputs 50R to the scope?

From probe datasheet:
" Both N2792A and N2793A probe are compatible with any oscilloscope with 50Ω BNC inputs"

Switch the scope to 50ohm, or use a 50ohm terminator between probe and scope.

 

From probe datasheet:
" Both N2792A and N2793A probe are compatible with any oscilloscope with 50Ω BNC inputs"

Switch the scope to 50ohm, or use a 50ohm terminator between probe and scope.

thank you Irving and Albert. I seem to have got confused with the input impedance of the probe and the scope.

 

It could be a setup issue such that the scope is set to report Peak to Peak while some other place it thinks it is reading Peak volts. Or Vice-Versus. Units display setup can lead to a lot of confusion if there are errors. So it might not be a hardware issue at all.
Usually hardware errors are just a small percentage off.

 

It could be a setup issue such that the scope is set to report Peak to Peak while some other place it thinks it is reading Peak volts. Or Vice-Versus. Units display setup can lead to a lot of confusion if there are errors. So it might not be a hardware issue at all.
Usually hardware errors are just a small percentage off.

Thanks Bill for this input - I did think it could be that as I’ve had same issue before. Fortunately setting the scope impedance to 50R resolved the issue though.