9 Layout

9.1 Layout Guidelines

A minimum of four layers is required to accomplish a low EMI PCB design (see Figure 9-1). Layer stacking

should 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 introduction of their

inductances) 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

• 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 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, see the Digital Isolator Design Guide (SLLA284)

9.1.1 PCB Material

For digital circuit boards operating at less than 150 Mbps, (or rise and fall times greater than 1 ns), and trace

lengths of up to 10 inches, use standard FR-4 UL94V-0 printed circuit board. This PCB is preferred over cheaper

alternatives because of lower dielectric losses at high frequencies, less moisture absorption, greater strength

and stiffness, and the self-extinguishing flammability-characteristics.

9.2 Layout Example

10 mils

10 mils

40 mils

FR-4

Keep this

space free

from planes,

traces, pads,

and vias

Ground plane

Power plane

Low-speed traces

High-speed traces

Figure 9-1. Recommended Layer Stack

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