The MMCM feedback can be internal to the MMCM when the MMCM is used as a synthesizer or jitter filter and there is no required phase relationship between the MMCM input clock and the MMCM output clock. The feedback clock is not subjected to noise on the core supply because it never passes through a block powered by this supply. Thus the MMCM performance increases. However, noise introduced on the CLKIN signal and the BUFG are still present (see the following figure).
Figure 1. MMCM with Internal Feedback - No Deskew
