Applied Fft to my temperature sensor readings and don't understand the result

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Hi everybody,
I'm reading data from my temperature sensor with arduino at a sampling frequency of 100Hz. Arduino writes data to the serial port and Matlab gets it perfectly (no data is missing). When I do the fft to see at which frequency I'm getting this result.
Second image is the zoomed graph of the 1st.
I'm new to digital signal processing, so I'm a bit confused: shouldn't DC gain be negligible? Is there an explanation for this result, or is it wrong.
Time series is plotted here also
Thanks in advance

Accepted Answer

Christoph F.
Christoph F. on 5 Mar 2018
> Is there an explanation for this result, or is it wrong.
The result looks correct. The Fourier transform shows a large DC component, because the signal has a large DC component.
> I'm new to digital signal processing, so I'm a bit confused: shouldn't DC gain be negligible?
What do you mean by "DC gain"? From the graph, it looks like the average value (-> DC component) is about 32.75°C, and all other variations are in the +/- 2.5°C range. A large DC component in the frequency domain correctly represents the temperature data.

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