12.1 Layout Guidelines

A minimum of four layers are required to accomplish a low EMI PCB design (see Figure 23). Layer stacking should be in the following order (top-to-bottom): high-speed signal layer, ground plane, power plane and low-frequency signal layer.

• Route the high-speed traces on the top layer to avoid the use of vias (and the introduction of the inductances) and allow for clean interconnects between the isolator and the transmitter and receiver circuits of the data link.
• Place a solid ground plane next to the high-speed signal layer to establish controlled impedance for transmission line interconnects and provide an excellent low-inductance path for the return current flow.
• Place the power plane next to the ground plane to create additional high-frequency bypass capacitance of approximately 100 pF/in².
• Route the slower speed control signals on the bottom layer to allow 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 system to the stack to keep it symmetrical. Adding a second plane system to the stack makes the stack mechanically stable and prevents it from warping. 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.