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confused by sigma-delta ADC

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enjoypercy

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I designed a 2nd order sigma-delda ADC in matlab.but I don't know why the power spectrum of noise is so bad.could someone point out the problems? I would appriciate any help.here is my code:
k=100; %oversampling rate
fs=200; %the nyquist frequence
t=0:1/(k*fs):1;
x=sin(2*pi*100*t);

v1(1)=0; %2nd order ADC
v2(1)=0;
u2(1)=-1;
y(1)=1;
for n=2:length(x)
v2(n)=v2(n-1)+u2(n-1);
if v2(n)>=0
y(n)=1;
else y(n)=-1;
end
e(n)=y(n)-v2(n-1);
u1(n)=x(n)-y(n);
v1(n)=v1(n-1)+u1(n);
u2(n)=v1(n)-y(n);
end



y_pinpu=20*log10(abs(fft(y)).^2/length(y));%/max(abs(fft(y))));
figure;
plot((1:200)/length(y_pinpu)*k*fs,y_pinpu(i));



how can I get the right result? thanks a lot!
 

maybe you need more periods to analysis!
 

can you show me the simulation result, I mean the waveform. that will be helpful to find the problem. :)
 
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    tlucky

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thanks for your attention!
I have sovled the problem.
the amplitude of the source signal can't be 1, which equals to the Vref. that will lead to the unstable state.
but is that the rule of ADC?
 

enjoypercy said:
thanks for your attention!
I have sovled the problem.
the amplitude of the source signal can't be 1, which equals to the Vref. that will lead to the unstable state.
but is that the rule of ADC?

for delta sigma adc, large input will overload the quantizer.
the input range is often limited.
 

Hi mate,
Do you have any code for ADC/DAC simulation ?
 

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