WaveForms + Analog Discovery 2: Changing the time base (or range) in the scope changes the signals amplitude

[MaWa]

I have a Digilent Analog Discovery 2. I want to use the scope function to measure a phase of a BLDC motor. For powering the drive controller I use a 24V battery. I noticed that in the scope when I change the time base or the range of the channel the amlpitude of the signale changes. When I use the logger function it shows about 12V in True RMS mode ("Logger.jpg") which seems reasonable due to a controll measurement with a multimeter.

 

I made some  screenshots to clarify / show what I mean:

"Tme base 1.jpg" to "Time base 4.jpg" show how the amlitude changes when changing the time base. The amlitudes are also way off from what the logger gives me. When I "zoom in" to get more details the amlitude really goes crazy ("Time base 4.jpg"). These are all with the same settings except of the time base.

"Range.jpg" shows what happened after changing the range (compared to "Time base 3.jpg). The singal is just visible by the small yellow dots on the top left of the scope screen.

I wonder what happens there. I don't think that's a problem in the software else I'd wonder that noone else discovered this before me. So not sure if this is a settings problem or what's going on.

I'd really want to make this work because I really like the device. Used the logging function a lot before without problems...

[attila]

Hi @MaWa

In Scope with Repeated and Scan modes changing the time base will adjust the sample rate.
By default the channels use average sampling mode which hides higher frequency components than the sampling rate.
The Noise option (under Time Options or each Channel) that you have disabled would have shown you these.
The signals may be clipped above 45-50V so the measurements won't give accurate result.
The signals go above 50V which could damage the device.

[MaWa]

Hi @attila

Thanks for that information. Luckily I bought the Discovery BNC, too. Now with my 10x probe I can measure as expected. The signals maximum amplitude equals the battery voltage what I expected. Didn't think about noise with possible higher ampltudes.