I think the fundamental parts of Delta-Sigma Design are "Digital Signal Processing" (Fiilter/Sampling Theorem/Spectrum/FFT) and "Noise-Shaping". Therefore, the best way to learn it is to build a behavioral-model on high-level (S/Z-domain mathematical viewpoint). Both simulink and open-source system-level toolbox are good tools for studying delta-sigma modulator for either ADC engineers or PLL designers.