Analog Discovery 2 issue

[rasfh]

I have the Analog Discovery 2 and the bnc board with scope probes. But I have problem with the 10X settings on the probes. The problem is that I get a little dc offset with the osciloscope and the built in functiongenerator. When I apply a sinus signal with an amplitude of 1 V, then I measure about 0,8 - 0,9 V positive voltage and negative 1,1 - 1,2 V, which means that there is a dc offset. The dc and ac coupling does not help. There is no problem with the 1X setting on the probes. I have compensated the probes on the 10X setting with a 1 KHz squarewave which have an amplitude on 1 V. What could be wrong? (: When I apply a dc signal with the functiongenerator, then I measure - 0,1 V on the osciloscope on 10X setting.

Best regards Rasmus 

[rasfh]

1 KHz sinewave with an 1 V amplitude in 10X setting

[rasfh]

Apply a dc signal, 10X setting 

[rasfh]

squarewave 1 KHz. 10X setting 

[rasfh]

Squarewave 1 KHz. 1X setting 

[attila]

Hi @rasfh

With 10x probe the scope input range is extended by 10, 5Vpk2pk changed to 50Vpk2pk, the resolution is reduced by 10, the offset error is increased by 10.
The offset can be quickly normalized using the "Zero offset" option under Time options or under each channel gear dropdown menu Average button. 

 

Your probe looks to be well tuned, but here you have information about this:

 

[rasfh]

@attila Thanks (: I set it to -50 mV which helped a bit. do I need to set that every time? The waveform is still jumping up and down (triggering not that good. Unstable) forgot to tell that last time. But the 1X setting does not have that problem. 

[rasfh]

My guess would be bad probes, and I don't understand why there can be a offset when I use AC coupling on the bnc board. 

[rasfh]

I also tried to use the zero offset funktion on the 1X setting as well, which give a offset of -1.31 mV

[attila]

Hi @rasfh

The "Zero offset" can be used to eliminate the device offset error (like do to temperature, which is also visible with AC coupling) and external minor DC component.

In case the signal is jumping, as you said, it could be a faulty probe. The probe switch is actually a mechanical switch which shorts the 9M and trimmer for 1x, and mechanics can wear out.

[rasfh]

@attila Allright, but the offset error could it maybe be fixed with a better quality probe?

[attila]

Hi @rasfh

The 10x probe should not introduce offset error. What is happening that the device offset error is multiplied by 10 due extending the range by 10 times.
Like here the 0.3mV 1x error get to 3mV.

This can be solved with calibration:

[rasfh]

@attila  Okay, should I use this guide to calibrate the device? 

https://www.instructables.com/id/How-to-Calibrate-the-Analog-Discovery-2/

Or should it be a hardware based calibration? 

I know about the zero offset function, But it is annoying to do every time you know. 

[attila]

Hi @rasfh

I don't recommend using the Calibration Wizard it can take long.
It is suffice to click on the links in the Calibration dialog to tune the required instruments (like Waveform Generator 1 Low/High Gain and Oscilloscope) and follow the instructions shown the application.

[rasfh]

@attila I did a full calibration, and now I am getting about 1-2 mV on the 10X setting with the zero offset function, which I believe is much better (: