hi,all
I have made a cascade2-1 sigma delta modulator as showed in the attached picture.Where parameters are:a1=a2=0.5,b1=2,c1=0.5,a3=0.5.
The sampling frequency is 1Mhz,input signal is 0.2sin(2*pi*200).But the output bitstream amplitude is (+2,-2),This is linux matlab simulation result. Surprisingly, on my windows matlab, some of the output bitstreams has exceeded (+20,-20). If i change the parameter c1=1,thus 1/c1 is 1 too, the bit stream is (+1 -1). The PSD of it not embody the third order noise shapping effect at all,and the SNDR is very low(36dB,OSR=128).
   I have readed some papers about Sigma Delta modeling, but all the examples are sigle stage,one quantizer.
   Anybody can help me to solve this problem, or is there mash sigma delta modulator's simulink model? any advice is appreciative.Thanks!

Best Regards!

hi,chungmnig
   Thanks a lot for your reply.I have modified my matlab program,I calculate the final output's SNDR,it also reached more than 100dB.
   The swing of integrator is not very large.But in logic cancellation,because not realized with digital model, it's output swing is very high,for example,through the gain of 2 after comparator model, the output swing becomes (2,-2).Have you look into the output streams, the amplitude is very high.Is this has some imfluences on system's performance?
   I felt in practice, this phenomena will be avioded for the digital realizition with digital adder,shifter and so on,so the swing will be not more than (vdd,0),ie (1,-1) in simulink model.But in simulink, ........Are there any methods to solve this? Thank you very much.