Hi all,

The digital sigma delta modulator is a 3rd order mash 1-1-1 digital modulator and the output range is -3~4. The operation frequency is fs. What I care is the output noise magnitude at N*fs/2.
Firstly, I did a transient simulation and do FFT for the sdm output number. By observing the spectrum, the magnitude points at N*fs/2 in decibel has a -20dB/dec slope.
On the other hand, however, if I calculate the spectrum in frequency domain in Mathcad. The magnitude values at N*fs/2 have no difference.
Could anyone help me explain this? Did I miss something?

Thank you very much!
-Ryan