Hello,

I've read around extensively about FFT setup for a sigma delta ADC and I'm not sure if I'm not setting it up properly .

my current setup for a first order sigma delta ADC

sampling frequency : 38.4 kHz (for the ADC and FFT setup)
FFT bins : 65536
input signal : (31*38400)/65536 = 18.1640625 Hz

I'm using Mentor Graphics Eldo for the simulation

start time : 0.578064516 s (which is 10 and a half cycles of the input)
stop time : 2.284735711 s (31 cycles of the input from start time)
window : hanning alpha = 0.5

below is the output FFT of the ouput however the noise floor is extremely high. the x-axis is in log scale. there seems to be noise shaping however the harmonics are quite high so I wanted to
[url="http://i.imgur.com/tPVjNAN.png"]http://i.imgur.com/tPVjNAN.png
[/url]

plotting the wave using matlab yields the same output

y = csvread('file4.csv');
d = y(:,2);
d = hann(65536).*d(1:65536);
z = abs(fft(d));
x = 20*log10(z);
semilogx(x);
[url="http://i.imgur.com/2UvzmHnm.png"]http://i.imgur.com/2UvzmHnm.png
[/url]

Thank you in advance for your help, extremely appreciated