Thanks for responding to my question. The annwer for your questions are: Measurment time : 300seconds, Sampling Frequency : 2 Hertz, and Window Type : Hanning. Hope you could hint me on this soon.

Question

Thanks for responding to my question. The annwer for your questions are: Measurment time : 300seconds, Sampling Frequency : 2 Hertz, and Window Type : Hanning. Hope you could hint me on this soon.

Answer 1

ok, I see you posted parts of your VI in a different location. There is a sub-VI missing though ( Spline Interpolate.vi). But I re-constructed your data and can see the following things:
1 - Your signal has a significant DC-offset (approximatelly 1000) and this will result in a power value for DC of approximatelly 10E6 (though you wrote 10E8 in your first posting).
2 - If you ignore that DC, you can see on the time signal that there is another signal at very low frequency (order of magnitude amplitude = 10, power spectrum around bin 1 = 100) and you are not capturing an entire period of that signal, so either a measurement time of 300 seconds is not enough or this is a signal caused by a drift that you want to ignore.
But overall there is not much sign
al left once you remove your DC and eventually the very low frequency signal. But given your input signal, the power spectrum looks right. Remember also that the DC component will be replicated (with 6 dB reduction) on bin 1 when you apply a Hanning window. Try eventually without the Hanning window (but you'll still have to ignore your DC bin). Alternatively subtract the dc value from your time signal before computing your power spectrum.