AN 958: Board Design Guidelines

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ID 683073
Date 1/28/2022
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5.3. EMI

Electromagnetic interference (EMI) is directly proportional to the change in current or voltage with respect to time. EMI is also directly proportional to the series inductance of the circuit. Every PCB generates EMI.

Precautions such as minimizing crosstalk, proper grounding, and proper layer stack-up can significantly reduce EMI problems.

Place each signal layer in between the ground plane and power plane. Inductance is directly proportional to the distance an electric charge has to cover from the source of an electric charge to ground. As the distance gets shorter, the inductance becomes smaller as well. Therefore, placing ground planes close to a signal source reduces inductance and helps contain EMI. Figure 83 shows an example of an eight-layer stack-up. In the stack-up, the stripline signal layers are the quietest because they are centered by power and GND planes. A solid ground plane next to the power plane creates distribution capacitances with low equivalent series inductance (ESL). With IC edge rates becoming faster, these techniques help to contain EMI.

Figure 83. Example Eight-Layer Stack-Up
Component selection and proper placement on the board is very important to controlling EMI. The following guidelines can help reduce EMI:
  • Select low-inductance components, such as surface mount capacitors with low ESR and ESL
  • Provide good return path to minimize lop inductance
  • Use solid ground planes next to power planes
  • Use adjacent ground planes next to each segmented power plane for analog and digital circuits

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