The target impedance of the PLL0VDDA plane is calculated as 16.5 Ω based on the values (see Power Supply):

The impedance of the place (Z) must be 16.5 Ω or less. For more information about ripples and its values, see DS0128: IGLOO2 and SmartFusion2 Datasheet. Plane impedance with and without filter circuit is shown in the following figure.

Figure 1. PLL0VDDA Plane Impedance