My nonideal Sigma Delta Modulator shows better SNDR in MATLAB than the ideal one?????

Dear engineers,
I have a really important problem!I made a second order Sigma delta modulator in MATLAB Simulink(OSR=128,vin=0.7,fin=1.5625KHz,fs=12.8MHz),I modeled the effect of Slew-rate and GBW in Simulink!when I ask matlab to calculate SNR of the ideal one, it gives 107.1dB(by hand calculation I get SNR=111.4dB) but when I add the Slew rate and GBW effect as a block in the forward path of the input signal,Matlab calculates SNR as 107.5(SR=50v/microsec & GBW=40 MHz),interestingly when I lower the Slew rate,it increases to 108.2dB,I''m really confused!I did this with one nonideal model of mine and with a modle from malobertie's book, and I got almost the same result!Now my problem is why my SNR increases with limited Slew rate and GBW?I think my model is right,because when I limit the slew rate and GBW further more it shows a decrease in the SNR!Is there a problem with my Simulation Configuration in Simulink?I''ll really be glad if anybody can help me with this,It''s almost pushing me out of my mind!:x

I assume that slew rate and limited bandwidth work towards a higher modulator order, thus slightly increase noise shaping. Stability of the modulator has to be checked, however.

Dear FvM,You know when the Opamp Slews with infinity slope and the GBW is so high making taw___>0 then it'll be like the Ideal case,but my problem is that when I give Slew rate and GBW mentioned above,I get a result better thn Ideal case!I really don''t know why?But I think there is a problem with my simulation configuration parameters, I just tried it again,when in Ideal case I put a gain block of 0.98 in the forward path in the ideal integrator before the 1/z delay,I get 108.3 in Ideal case?Maybe this will help you understand my problem better!anyway thanks for your answer,

I guess, you didn't yet understand my reference to modulator order, how changing the modulator transfer function affects SNR. It's of course a bit more complicated for slew rate, because the effect is beyound linear system description.

So you mean that What I get as answer is correct?

So you mean that What I get as answer is correct?

Click to expand...

Yes, it sounds plausible.