A minimum of two layers is required to accomplish a cost optimized and low EMI PCB design. To further improve EMI, a four layer board can be used (see Figure 8-7). Layer stacking for a four layer board must be in the following order (top-to-bottom): high-speed signal layer, ground plane, power plane and low-frequency signal layer.

• Routing the high-speed traces on the top layer avoids the use of vias (and the related parasitic inductance) and allows for clean interconnects between the isolator and the transmitter and receiver circuits of the data link.
• Placing a solid ground plane next to the high-speed signal layer establishes controlled impedance for transmission line interconnects and provides an excellent low-inductance path for the return current flow.
• Placing the power plane next to the ground plane creates additional high-frequency bypass capacitance of approximately 100pF/inch².
• Routing the slower speed control signals on the bottom layer allows for greater flexibility as these signal links usually have margin to tolerate discontinuities such as vias.

If an additional supply voltage plane or signal layer is needed, add a second power or ground plane to the stack keeping the layers symmetrical. This makes the stack mechanically stable and prevents warping. The power and ground plane of each power domain can be placed closer together, thus increasing the high-frequency bypass capacitance.

For detailed layout recommendations, refer to the Digital Isolator Design Guide.