A minimum of two layers is required to accomplish a low EMI PCB design. To further improve EMI, a four layer board can be used (see Figure 11-2). Layer stacking for a four layer board should be in the following order (top-to-bottom): high-speed signal layer, ground plane, power plane and low-frequency signal layer.
If an additional supply voltage plane or signal layer is needed, add a second power or ground plane system to the stack to keep it symmetrical. This makes the stack mechanically stable and prevents it from warping. Also the power and ground plane of each power system can be placed closer together, thus increasing the high-frequency bypass capacitance significantly.
For detailed layout recommendations, refer to the Digital Isolator Design Guide.