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Some questions about Sigma-delta modulator

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lunbaby

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I design a 3-order sigma delta modulator for audio application and want to achieve
16-bits resolution(96dB). For hspice simulation, I write FFT code :

fs=5.12MHz BW=20k fin=fs*N/np , N is prime

.tran 1/fs 12779.62us
.fft v(y) start=100ns stop=12779.62us window=hann np=65536 freq=fin

freq: input frequency
start: choose arbitrarily
stop: start + np*(1/fs)


I use 1/fs for the scale the PSD is only 2-order shaping(40db/dec)
Change the 1/fs to 1ns(more smaller) the PSD is better!!

But when I want to calculate FFT in matlab, the output points are much more than
np(65536) so I don't understand how to do.

And which one can tell me why using the scale 1/fs can't achieve good result ?
Thanks~~!!
 

I had been simulated SDM & calculated FFT through MATLAB.

There must be 65536/2 point when you see FFT plot between 0~ fs/2 if NP=65536

I think you put your spice output data to MATLAB with the range of 0~fs instead of 0~fs/2.

If i'm wrong, write more.
And I want to know your MATLAB code.

Regards.
 

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