CC BY-SA 2.0 by Miguel
Electromagnetic interference (EMI) is electromagnetic energy that disrupts the signaling in an electronic device through radiation or induction. From the static on your radio to that buzz you hear when you bring a cell phone near audio equipment, EMI is all around us.
To create EMI, all you need is energy and an antenna. Electronic devices are basically boards teeming with electromagnetic energy. All you need is a dipole across an energy source, and you’ve got an antenna that can radiate EMI. What’s more, because EMI radiation can affect critical electronics in the world around us, such as the equipment on an airplane, there are EMI/EMC regulations and standards you have to pass before your product can go to market. Let’s dive into the common techniques you can employ to reduce EMI in your PCB designs.
The ground plane is in many ways your first line of defense against the kind of noise produced by EMI, because circuits require at least a floating ground in order to work. To use the textbook water analogy, if the current in your circuit is like water in a series of water pipes, the ground plane is like a large basin of water. As the water pours into the basin, ripples are created. Those ripples are easily absorbed by a large basin, but if the basin were smaller, they’ll stay around for longer and bounce around the walls of the basin.
In a PCB, the ground plane is there to provide a 0 volt reference line to the power supply ground terminal for the return path of a circuit. Unlike the basin, however, when “ripples” are made, noise is produced, dipoles can form and the entire board can become an antenna. This is why the ground plane, that copper foil layer in your PCB, takes up as much of the cross-sectional area of the board as possible. Reducing EMI across your board starts with how effectively you utilize the ground plane. Some common best practices for reducing EMI with ground include:
Use multi-layer board. Ground plane too small? Adding another layer can give you more options on how to handle high speed traces on your board. Differential pairs generating crosstalk? Route them in an inner layer where the noise is lowered.
Use split ground planes with caution. If you’re going to split the ground plane, make sure you have a good reason, such as to separate analog and digital grounds to avoid noise coupling, because split ground planes can act as slot antennas and radiate.
Only connect split ground planes at a single point. The more common ground connections you have, the more loops you create, and the more EMI your design will radiate.
Connect bypass or decoupling capacitors to ground plane. If you have any of these in your
design, you can reduce the return current path by connecting them to ground, which reduces the size of the loop, and therefore radiation. Just be sure not to connect a bypass capacitor between a power plane and an unrelated ground plane, which can cause capacitive coupling.
Traces are conductive paths on a board, which contain flowing electrons while a circuit is active—that means they are just a bend or cross away from creating a fully radiating antenna.
Common best practices for trace layout include:
Avoid sharp right-angle bends. Capacitance increases in the 45° corner region changing the characteristic impedance and leading to reflections. This can be mitigated by rounding right angles.
Keep your signals separate. Keep high speed traces (e.g. clock signals) separate from low speed signals, and analog signals separate from digital signals.
Keep return paths short.
Route differential traces as close as possible. This increases the coupling factor, bringing influenced noise into the common mode which is less problematic for a differential input stage.
Use vias wisely. Vias are necessary because they let you take advantage of multiple layers in your boards when routing. Designers must be aware that they add their own inductance and capacitance effects to the mix, and reflections can occur from a change in characteristic impedance.
Avoid using vias in differential traces. If you must, use an oval anti-pad shared by the two vias to reduce parasitic capacitance.
Arrangement of components
Electronic components are the building blocks of electronic circuit. Being mindful of the EMI impact of each component can lead to better PCB design. Best practices for component layout include:
Keep analog circuits separate from digital circuits. As with the traces, AC and DC circuits should be kept apart to avoid crosstalk and other issues. Shielding, taking advantage of multiple layers, and using separate grounds are all viable options.
Isolate high speed components. The faster and smaller the component, the greater the EMI. Mitigate the natural EMI effects of high frequency clocks in CPUs and GPUs with shielding and filtering.
Some components will inevitably produce EMI, and that’s okay. We can shield them with a Faraday cage—an enclosure made of conductive materials with sufficient thickness to block RF waves. While the ideal Faraday cage would be a conductive enclosure with no openings, in practice we use boxes made of metal or conductive foam called gaskets.
Reduce EMI in Your Next PCB Design
It’s all too easy to accidentally create an antenna in the way you layout the traces and vias on your board. Couple that with the ever rising demand for higher clock speeds, and you begin to realize why reducing EMI is more important than ever before. Ready to start incorporating some of these EMI reduction tips into your next design? Check out Cadence’s suite of PCB design and analysis tools today.
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