amplitude axis of frequency domain plot not matching question tia sal22

[Rick T]

Greetings all 

I have a frequency domain plot that plots but the amplitude doesn't match 

the equation at all.  In the example I have the amplitude set to 8 but when I plot it the amplitude

shows up as 30+ why is this?

tia sal22

code below:

Fs = 1000;                    % Sampling frequency

t=linspace(0,1,1000);

vp_sig_orig=8*sin(2*pi*2*t)

vp_sig_len=length(vp_sig_orig); 

% Use next highest power of 2 greater than or equal to length(x) to calculate FFT.

nfft= 2^(nextpow2(length(vp_sig_orig))); 

% Take fft, padding with zeros so that length(fftx) is equal to nfft 

fftx = fft(vp_sig_orig,nfft); 

% Calculate the numberof unique points

NumUniquePts = ceil((nfft+1)/2); 

% FFT is symmetric, throw away second half 

fftx = fftx(1:NumUniquePts); 

% Take the magnitude of fft of x and scale the fft so that it is not a function of the length of x

mx = abs(fftx)/length(vp_sig_orig); 

% Take the square of the magnitude of fft of x. 

mx = mx.^2; 

% Since we dropped half the FFT, we multiply mx by 2 to keep the same energy.

% The DC component and Nyquist component, if it exists, are unique and should not be multiplied by 2.

if rem(nfft, 2) % odd nfft excludes Nyquist point

  mx(2:end) = mx(2:end)*2;

else

  mx(2:end -1) = mx(2:end -1)*2;

end

% This is an evenly spaced frequency vector with NumUniquePts points. 

freq_vect = (0:NumUniquePts-1)*vp_sig_len/nfft; 

% Generate the plot, title and labels. 

subplot(2,1,1),plot(t,vp_sig_orig)

title('Time domain plot'); 

xlabel('time'); 

ylabel('Power'); 

plot_len=1000;%length to plot

subplot(2,1,2), plot(freq_vect(1:end),mx(1:end))

title('Power Spectrum of vp'); 

xlabel('Frequency (Hz)'); 

ylabel('Power');