Finding Accurate Amplitude Using FFT

 I have been working on code to use the FFT to find the amplitude of a signal. However, despite my best efforts to window I seem to get incorrect amplitudes. When I manually find Amplitude by inspecting the signal I get A = 63.7. When I use the following code I get A = 54.8. This is causing significant errors in my Bode plot.

I tried using a flat top window to maximize amplitude accuracy. My signal does not seem significantly noisy. I expected an amplitude much closer to the true value.
function Amplitude = Gaindb(x)

 if size(x, 2) > 1
    x = x';

 x = x - mean(x);

 N = length(x);
 win = flattopwin(N);

 x = fft(x.*flattopwin(N));

 Amplitude = 2*max(abs(x))/N;

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When I run your code I get 14.5 for the answer, so I am not sure how you are getting 54.8. But the 14.5 is consistent with the Matlab definition of the flattop window. From the documentation, the flattop is a sum of cosines with certain coefficients, along with a constant coefficient a0 = .216. a0, the area of the window, is basically the amplitude reduction factor for the signal.
Without any windowing of x, the fft comes up with a signal amplitude of 67.1, and 67.1 x .216 = 14.5 which is consistent.
The Matlab window peaks out at a value of 1.


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