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gelokk
Greece
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 Posted - 12/23/2005 : 05:42:06 AM
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Origin Version (Select Help-->About Origin): 7.5
Operating System: Win XP
Hello, I have a question regarding Origin's exponential decay fitting. I have some data that I'd like to fit, so I plot them and then change y-axis to log scale, so my graph appears as a straight line (exponential data appear as line when you plot them in log scale in the y-axis and I need my graph to look like that)
I then fit them using: Analysis - Fit Exponential Decay - First Order, but strangely the fit is a curved line and not a straight line. Shouldn't the fit be a straight line? (remember I'm using a log scale in the y-axis).
The strange thing is that if I do the same procedure manually through the Advanced Fitting Tool (i.e. define a new exponential decay function myself and fit the data), then the fit does appear as a straight line as it should.
Am I doing something wrong or is that an error of Origin's automated exponential decay fit?
Thanks very much for your help.
Antony
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easwar
USA
1964 Posts |
Posted - 12/23/2005 : 12:18:16 PM
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Hi Antony,
The difference should be due to the y0 offset term. The default exp functions in Origin are of the form
y = y0 + Ampl * Exp (-x/t)
and so if you take the log, a non-zero y0 value will make the line to be not straight.
When you defined a new exp function, you probably did not have an offset term?
Easwar
OriginLab
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gelokk
Greece
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Posted - 12/24/2005 : 06:20:43 AM
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Hi Easwar,
Yes, that was it! You are right, it's the offset term that makes the line not straight, and I didn't have one in my defined function.
Thanks very much for your help! |
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Exponential Decay