Three layers are sufficient to accomplish a low EMI PCB design. Layer stacking should be in the following order (top-to-bottom): high-speed signal layer, ground plane, optional power layer, and low-frequency signal layer.

• Routing the high-speed traces on the top layer avoids the use of vias (and the introduction of their inductances) and allows for clean interconnects between the isolator and the transmitter and receiver circuits of the data link.
• For best performance, it is recommended to minimize the length of D+/D- board traces from the MCU to ISOUSB211-Q1, and from ISOUSB211-Q1 to the connector. Vias and stubs on D+/D- lines must be avoided. This is especially important for High Speed Operation.
• Placing a solid ground plane just below the high-speed signal layer establishes controlled impedance for transmission line interconnects and provides an excellent low-inductance path for the return current flow. D+ and D- traces must be designed for 90-Ω differential impedance and as close to 45-Ω single ended impedance as possible.
• Placing the power plane next to the ground plane creates additional high-frequency bypass capacitance of approximately 100 pF/in².
• Decoupling capacitors must be placed on the top layer, and the routing between the capacitors and the corresponding to supply and ground pins must be completed in the top layer itself. There should not be any vias in the routing path between the decoupling capacitors and the corresponding supply and ground pins.
• ESD structures must be placed on the top layer, close to the connector, and right on the D+/D- traces without vias. Ground routing for the ESD structures must be made in the top layer if possible, else must have a strong connection to the ground plane with multiple vias.
• 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.
• Connect a small plane (for example, 2 mm x 2 mm) to the GND pins on the top layer to improve thermal performance. Connect this to the ground player in the second layer with multiple vias. See Layout Example for details.