Richard Schreier's Matlab toolbox for the design and analysis of sigma-delta modulators can be downloaded for free from here. It works with R14 and higher.

If you ever work on the actual design of a sigma-delta modulator, this toolbox is immensely useful. It synthesizes the modulator given a noise transfer function, simulates the modulator for a given input, and also gives you realizable structures to implement the modulator. Plus a whole lot of other neat things. It comes with a very well-documented help file.