Difference between LabVIEW FFT and Matlab FFT

[DanB1983]

Does anyone know why there is an offset in the absolute levels of the results of the FFT function in LabVIEW and Matlab?

See the attached VI for an example of my LV code.

Also attached is the plot generated in Matlab, which seems to be offset by ~37dB.

This was generated using the command 'plot(10*log10(abs(fft(x))))', where x is the same input array as in the VI.

Many thanks,

Dan

[Darin.K · Accepted solution]

I see that you are using the FFT Spectrum VI which returns a single sided spectrum.  To convert from a two sided transform (ie FFT) to a single sided spectrum with RMS units there is a normalization of sqrt(2)/N where N is the number of points in x(t).  You can premultiply your x(t) by N/sqrt(2) and get the same results as Matlab.  Or, you could use the FFT.vi so you are comparing transform to transform instead of spectrum to transform.

[canyon]

Stupid question but in your Vi final the waveform graph on the Control Panel shows up as a double in the programming window. When I make waveform graphs they end up being a little graph in the programming window. What's the difference if there is one?