8.4 Layout

8.4.1 Layout Guidelines

A minimum of four layers is required to accomplish a low EMI PCB design (see Figure 8-4). Layer stacking 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 introduction of the

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

approximately 100pF/inch2.

• 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.

If an additional supply voltage plane or signal layer is needed, add a second power or ground plane system

to the stack to keep the planes symmetrical. This makes the stack mechanically stable and prevents 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, refer to Digital Isolator Design Guide.

8.4.1.1 PCB Material

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

8.4.2 Layout Example

10 mils

10 mils

40 mils

FR-4

0r ~ 4.5

Keep this

space free

from planes,

traces, pads,

and vias

Ground plane

Power plane

Low-speed traces

High-speed traces

Figure 8-4. Layout Example Schematic

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ISO7840, ISO7840F

SLLSEN2C – JULY 2015 – REVISED JULY 2025

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