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

[Emehdi]

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

 

[FvM]

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.

 

[Emehdi]

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,

 

[FvM]

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.

 

[Emehdi]

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

 

[FvM]

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

Yes, it sounds plausible.