Oscilloscope Loading

Discussion in 'Homework Help' started by Biomed27, Jan 30, 2015.

  1. Biomed27

    Thread Starter New Member

    Jan 30, 2015
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    The vertical amplifier of most oscilloscopes typically has an input resistance of 1MOhm and a calibration accuracy of 3% of full screen deflection. You want to use a scope to measure voltages in a variety of circuits yet limit loading by the scope to 3%. What is the highest output resistance of the circuits that you wish to measure that will remain within this load limit?

    I originally thought I could solve the problem by setting up a voltage divider solving for R1 and R2 for example. Upon writing some equations down I ended up with an R^2 term thus a nonlinear relationship and I decided I am making things too complicated. Maybe I don't fully understand the question.

    Does anyone have any suggestions?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes it's a simple voltage divider problem, so you determine what resistance in series with the 1 Mohm input will reduce the voltage by 3%.
    Of course, worst-case, that can give an overall error in the measurement of 6%. Personally I would want to limit the loading error to less than 1% to maximize the overall accuracy.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Work backwards. You want to limit the error to 3%.

    What value of R can you put in parallel with 1MΩ that will result in an equivalent resistance of 0.97R?
     
  4. Biomed27

    Thread Starter New Member

    Jan 30, 2015
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    I calculated about 32MOhm . . . But that doesn't seem right? If I set the parallel combination of 1MOhm and R equal to 0.97x1MOhm, thats the result I get for R . . .
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Try again. You made a mistake somewhere.
    Think about it.

    1MΩ in parallel with 1MΩ would be 500KΩ.

    1MΩ in parallel with 10kΩ would be close to 10kΩ to within 1%.
     
  6. Biomed27

    Thread Starter New Member

    Jan 30, 2015
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    Ok, I think I was misinterpretting the problem. Instead of setting the parallel combo equal to 970k I wanted to set it to 3% of 1MOhm = 30K, in this case using the 1Mohm value given and the calculating the value of the circuit resistance I end up with about 30kOhm, which makes much more sense because in that case the circuit resistance is much less than the oscilloscope therefore the oscilloscope will have a small loading effect.

    Thank you!
     
  7. Biomed27

    Thread Starter New Member

    Jan 30, 2015
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    Or more precisely, no greater than 30.9kOhm . . .
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    You got it!

    Now you can see how you can estimate this in your head.

    10kΩ would be accurate to 1% with a 1MΩ scope probe.
    20kΩ would be accurate to about 2%.
    30kΩ would be accurate to about 3%.

    Note that is approximate. It goes out of kilter as you go higher.
     
    Last edited: Jan 30, 2015
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