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Getting rid of noise in derivative?

  1. Jul 1, 2013 #1
    Hey guys, i'm working on some Current-Voltage data in excel. I need to do some analysis with the data, and the first step is to compute dV/dI. I have to do this in excel so decided to use excel's SLOPE function (5 point slope) to compute dV/dI. However, it seems that the derivative has a lot of noise, especially towards the lower values. Since I am writing a program to automate this analysis, I would really like to get rid of this noise.

    Another way I thought of was to fit a line to my initial data, and simply differentiate the fitted line's equation. However, I can't seem to get a decent fit for my initial curve.

    Pic shows the initial data and my horrible looking derivative.

    i_v_curve.jpg
    screenshot app
     
  2. jcsd
  3. Jul 1, 2013 #2

    Stephen Tashi

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    Science Advisor

    If you want a mathematically defensible way to smooth the derivative then you need to explain the nature of the "noise". in the data - e.g. can it be modeled as an independent random error at each measurement?

    Another thing to try is a "multi-point method" for estimating the derivative. For example, see the "Higher-order methods" section of http://en.wikipedia.org/wiki/Numerical_differentiation. (However, I don't know if that method has any "noise cancelation" properties.)

    If you only want a nice pitcure, Is there some reason that you can't fit a smooth function to the data first and then plot the derivative of that function?
     
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