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EMI Shielding is any electromagnetic radiation or disturbance which is emitted from an electrical circuit or electronics components, or electric circuits. EMI Shielding may be introduced intentionally as in the case of electronic warfare, or unintentionally from an electronic or electrical device or system. It may also be created as a result of a nuclear explosion. EMI Shielding should be an integral consideration to electronic device or electronics system design. Good EMI design techniques include, mechanical or electromagnetic shielding, careful circuit filtering design, adequate circuit ground design, as well as, attenuation to the bonding of the ground points. Packaging/Enclosure Design
Select enclosure materials absorb or block EMI Shielding. Consider metal shielding barriers with discontinuities. Within your electronic enclosure, allow for the installation of an EMI gasket device or materials. There are many types and configuration of gaskets or elastomers which help facilitate EMI sealing of your electronics enclosure. As required, design separate isolated areas within your electronics enclosure to separate or isolate noisy or sensitive electronics circuits and components. The separated electronics may be connected high impedance feedthroughs. PCB Design
When designing a PCB, the design goal is to control the following: - EMI Shielding from the PCB circuitry;
- Sensitiveness of the PCB circuits to EMI interference;
- Coupling between PCB circuits and external electronics systems;
- Coupling between circuits on the PCB.
PCB layout designers should minimize impedance discontinuities by use of low amplitude signals. If you have clock frequencies above 10 MHz, it may be necessary to design a multi-layer board with a aground layer. If this is not possible or is cost prohibitive for your product, try to use guard banding. Separate components so that noisy and sensitive circuits are placed as far away from each other as possible. Keep clock traces, buses and chip enables separate from I/O lines and connectors. Orient clock trace runs perpendicular to signal trace runs, and design the smallest possible trace width. If your clock devices are off the PCB, locate them close to the system connectors. If your clock devices are on the PC board, locate them so that they are central to the connecting components with the objective to minimize onboard distribution traces. Locate your input/output chips near the associated connectors. Consider using a resistor, inductor, or ferrite bead device to dampen the output circuits. Circuit types should be separated, as well as their grounds. Engineer's Edge. "Electronics, Instrumentation & Electrical Database." March 22, 2010. http://www.engineersedge.com/instrumentation/emi_rfi_shielding.htm
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