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I am new to signal processing and I have likely a very easy question but I have found inconsistent answers everywhere.

When performing an FFT, does it matter what units the amplitude is in? Currently my input is in mV.

If I use mV for an FFT, then use it to get a spectrogram, what are the visual units (strength of signal) of the spectrogram? Is it dB?

I'm using matplotlib.specgram to perform this.

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  • $\begingroup$ Spectrogram intensities will map to signal amplitudes with certain adjustments. $\endgroup$ Mar 16, 2023 at 10:32

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The FFT is an algorithm that computes the Discrete Fourier Transform of an input sequence from one domain (such as time) into an output sequence in the frequency domain.

The DFT does not care about units.

If your input domain is in mV, the FFT output is in mV also. Scaling these amplitudes is up to you, depending on what information you're interested in.
If you're asking what scaling should be used, that's a different question that begs for a lengthy answer, since there are lots of different ways to scale depending on the use-case.

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