hi,everyone
   I am designing a 5 CIFF delat sigma modulator,and i meet some problem when i go to simulation of circuit.There is my simulation situation:
input signal:sinusoidal signal:magnitude:1V,frequency:9.46KHZ;samoling clock:2.5MHz;Vdd=5V.and the circuit is full deffrential construction.
  i do the spectrum analysis to the output in candence.in the psd window,i set the time from 50.2u to 3327u,the sampling number is 8192;it just 31 cycles of input signal. the hanning window number is also 8192;then i got a good result of psd.
  but i got a bad result when i exported  to matlab,i used the same spectrum analysis situation:i used the fuction of calcSNR in matlab.it is as follow:
function [snrdB,ptotdB,psigdB,pnoisedB] = calcSNR(vout,f,fBL,fBH,w,N)
fBL=ceil(fBL);% fBL:                  Base-band lower limit frequency bins
fBH=ceil(fBH);% fBH:                  Base-band upper limit frequency bins
signal=(N/sum(w))*sinusx(vout(1:N).*w,f,N);      % Extracts sinusoidal signal
noise=vout(1:N)-signal;                              % Extracts noise components
stot=((abs(fft((vout(1:N).*w)'))).^2);            % Bitstream PSD
ssignal=(abs(fft((signal(1:N).*w)'))).^2;      % Signal PSD
snoise=(abs(fft((noise(1:N).*w)'))).^2;            % Noise PSD

pwsignal=sum(ssignal(fBL:fBH));                  % Signal power
pwnoise=sum(snoise(fBL:fBH));                    % Noise power

snr=pwsignal/pwnoise;
snrdB=dbp(snr);
norm=sum(stot(1:N/2))/sum(vout(1:N).^2)*N;      % PSD normalization
if nargout > 1
     ptot=stot/norm;
     ptotdB=dbp(ptot);
end

if nargout > 2
     psig=ssignal/norm;
     psigdB=dbp(psig);
end

if nargout > 3
     pnoise=snoise/norm;
     pnoisedB=dbp(pnoise);
end


can you give me some advice?thank you!