Three layers are sufficient to accomplish a
low EMI PCB design. Layer stacking must 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 the inductances) and allows for
clean interconnects between the isolator and the transmitter and receiver
circuits of the data link.
- For best performance, minimizing
the length of D+/D- board traces from the MCU to ISOUSB211, and from ISOUSB211 to the connector is
recommended. Vias and stubs on D+/D- lines must be avoided. Avoiding vias and stubs on D+/D- lines 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.
There must 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 typically 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.