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DSO6014A Oscilloscope ground floating

Question asked by scopeuser on Jun 13, 2012
Latest reply on Jun 14, 2012 by scopeuser

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Dear all,

We are having trouble in getting a ground signal (0V) when we connect to the ground on the scope. For some reason when we connect the scope to the internally generated square wave signal and to the scope ground, the ground seems to be floating at about 20mV.
In addition the ground level doesn't change even if the scope ground is connected to an external power-supply ground.

What could be the issue?

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algoss Jun 14, 2012 5:13 AM
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How do you know it's not a problem with the square wave?

I assume that you are using a scope probe. What happens if you connect the probe ground to the probe signal? If you are using a cable, what happens if you short the conductor to the shield (ground)?

Al
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- scopeuser @ algoss on Jun 14, 2012 6:37 AM
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  Thanks for your reply.
  
  Even if I connected the probe to the ground i.e.connect both the probe wires to the Scope ground, I still see the ground at about 20mV. In addition when I change the scale on the oscilloscope i.e. go from 10mV to say 100mV, the amount of offset on the ground signal varies.
  
  I am never able to see the ground level at 0V.
  
  I also connected the probe to the ground of an external power supply and it doesn't seem to make any difference. The offset was still there.
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  - algoss @ scopeuser on Jun 14, 2012 6:48 AM
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    First question, which I should have asked before: When was the last time you did a user calibration on the scope? I don't know the process for the 6000 series, but it might take care of the problem.
    
    I'm trying to figure out where the offset is.
    
    Are you using a scope probe?
    
    If so, forget connecting to the scope ground, what do you see when you connect the probe ground to the probe signal?
    
    Do you get the same results with different scope probes?
    
    If you take off the probe, and use a short (<10 cm) piece of wire to carefully short the inner conductor of the channel (signal) to the outer ring (ground), what do you see?
    
    If all of the above show consistent results, then the problem is in the scope. If a user calibration cycle doesn't fix it, then you need a repair.
    
    Al
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    - scopeuser @ algoss on Jun 14, 2012 11:34 AM
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      Hi Al,
      
      Thanks for your response.
      
      The instrument has not been calibrated in a long time. I will perform the user calibration and see if it gets fixed.
      
      Thanks.
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DSO6014A Oscilloscope ground floating

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