steady DC signal measurement changes when I change volts/div.

Discussion in 'General Electronics Chat' started by Mrdouble, Aug 2, 2015.

  1. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    My new oscilloscope is throwing me for a loop. When I measure a steady DC signal, the level changes when I change the volts/div. The software and graticle measurements both change to reflect the same results when I change volts/div. It seems to me, irrespective of the volt/div setting the voltage should always be the same. Probes are stock rigol probes that came with scope. Probes are selected 10x. I should say all measurements have been done on channel 2 and haven't tried channel 1. I have given the scope a self calibrate but results are the same. Thank you in advance.

    image1s_Mrdouble.jpg image2s_Mrdouble.jpg
    image3s_Mrdouble.jpg
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    This is what my old Rigol 1052E shows when I measure a DC 5.056V at least this is what my fluake 87V is showing.

    For 1V/div I have 5.05V

    NewFile2.PNG

    For 2V/div ----> 5.08V

    NewFile3.PNG

    For 5V/div ----> 5.7V

    NewFile4.PNG

    And finally surprise surprise for 100V/div Rigol measured 16V quite big error. And I do not know if this is normal or not.

    NewFile5.PNG
     
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  3. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    Dang :/. I only went down, not up
     
  4. #12

    Expert

    Nov 30, 2010
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    It may be normal, but I say it's bad...like, "send it back" bad.
    I can't tell you how to fix it or even where to look, but I sure can agree, it's completely unacceptable by my standards.
     
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  5. ScottWang

    Moderator

    Aug 23, 2012
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    You didn't compress the image, three files spent over 10 minutes to download, too much file size will affecting the connection speed for the forum, it is unnecessary, and it will also affecting the members to join to the discussion, I already compressed them from about 1.8MB reduced to 80KB, please compress the file resolution to about 800x600 or 1024x768 before you upload them, it will enough for the most circuits or images, thank you.
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    If you can't send it back, try to get the calibration or maintenance manual. As I recall on the old Tektronix CRT scopes that I used to calibrate there was a "balance" adjustment to the preamp for this very item. Maybe there is a procedure for your scope.

    Have you run the automatic calibration routine yet? (if there is one)
     
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  7. #12

    Expert

    Nov 30, 2010
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    I'm just too old to know this stuff. If my scope wanders, I clean the range switch.
    How do you clean a digitally selected range switch? :confused:

    Please let us know how this turns out. :)
     
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  8. dl324

    Distinguished Member

    Mar 30, 2015
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    I'd say you were making some invalid assumptions.

    On my Tek 7D20 (40 year old 70MHz digital scope plug-in for Tek 7K series analog scopes), the voltage readout is only accurate when the cursor is on a part of the waveform that's "on screen". When you change vertical sensitivity, you also change your resolution. For my scope, min resolution on the 5V/div scale is 200mV, on 2V/div it's 80mV, on 1V/div it's 40mV. When I switch vertical sensitivity, the displayed voltage is within the range resolution as long as the cursor is on screen. When it goes off screen, the reading is incorrect; but the solution is to select an appropriate range to display the waveform.

    BTW, I rarely use the on screen readouts; I prefer to look at the waveform and measure the voltage myself. I only use the on screen readouts to remind me of vertical and horizontal settings. That's probably because I started out using analog scopes and don't have much use for most of the features of a digital scope; though I have a half dozen of them...

    For my taste, your scope has too much information...
     
  9. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    I did run the self calibration and I must say it looked quite exhaustive. By the looks of it, all 4 channeles were compared against each other and bouncing all over screen Took like 30 mins. I'm not kidding.
     
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It is difficult to see where ground is in your pictures. I think I would stat out making sure 0 volts is on the screen - then measure.
    Also, what is "course" on the vots/div.?
     
  11. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    These were strait off my phone
     
  12. ScottWang

    Moderator

    Aug 23, 2012
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    You can find some compress apps.
     
  13. dl324

    Distinguished Member

    Mar 30, 2015
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    You just don't know how to use your scope:eek:

    The resolution on your 100V/div range appears to be 12V. If that's the case, the 16V measurement would be within the resolution of the scope on that range.

    A problem I see with digital scopes is that they give the appearance of more accuracy than one would reasonably expect.
     
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  14. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    Course/fine movement of volts/div knob. Is that what you mean?
    Not sure what you mean by ground in pic. Ground is below screen as indicated by arrow pointing down. Had to pull it low to fit signal on screen. I guess I could have just as easy used one volt signal lol
     
  15. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    I understand the resolution on higher volt/div but can you explain why on lower?
     
  16. ronv

    AAC Fanatic!

    Nov 12, 2008
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    That may be the problem. Try it with ground on the screen and the 2 and 5 volt scale.
     
  17. dl324

    Distinguished Member

    Mar 30, 2015
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    Can you describe how your scope handles vertical offset?

    On my scope, the center of the graticle is 0V. When I apply a vertical offset, the digital voltage readout reflects that offset. For example, on the 1V range, if I move the trace to the bottom of the screen; no input gives me a readout of -4V. If I measure the 4V from the calibrator, I get a readout of 0V at the peak (it's a 1KHz square wave).
     
  18. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    Mines exactly the same
     
  19. dl324

    Distinguished Member

    Mar 30, 2015
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    In that case, you probably need to add the vertical offset to the displayed voltage to get the actual voltage.

    What is Vpp? Is it range resolution?
     
  20. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    with signal at 5.56v my error is 12v which makes sense after what DL324 said about resolution.
     
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