Return currents in PCB designs are often misunderstood, yet they play a critical role in electromagnetic compatibility (EMC) and signal integrity, especially across low and high-speed signals.

Are you optimizing your layouts for these currents, or are hidden issues increasing EMI risks?

Our latest article, Effective Strategies for Managing EMI in PCBs with Low and High-Speed Signal Return Currents, explores how frequency impacts return currents and provides practical guidance for robust PCB design.

It reveals how to control return paths to minimize EMI and ensure reliable performance, whether you’re designing for low-frequency circuits or high-speed applications.

Why Read This?

A common PCB design mistake?

Placing components like the crystal oscillators too far from the pins they feed.

Crystal clock signals, with sharp edges, carry high-energy harmonics.

When traces are long, these harmonics can couple to nearby signals or the power net, spreading interference across or beyond the board, causing EMI issues.

The solution: position crystals as close as possible to their pins to shorten traces and minimize coupling risks.

Grounded? Think Again for EMI Control

A frequent design error causes electronics to fail EMC tests: pigtail connections between shielded cables and chassis.

Many assume grounding the shield prevents EMI, thinking, “Isn’t the shield now grounded?” Not exactly.

A single-point, high-impedance connection disrupts the Faraday cage effect intended to contain common mode currents.

This allows currents to escape as emissions or makes circuits susceptible to external interference, such as ESD (watch for my upcoming guide).

If you're using both layers for signals without a dedicated return and reference plane (RRP), you're very likely headed for EMC test failure.

Why?

Because radiated emissions from differential mode currents—that is, your normal operating current—are proportional to the area of the current loop.

Bigger current loop → bigger enclosed area → bigger emissions → bigger chance of failing EMC.

Are there exceptions that pass?

The warning against 90-degree bends in PCB traces is a staple of design guidelines, often blamed for signal integrity issues.

But is this caution rooted in fact, or is it an outdated myth?

Understanding the true impact of 90-degree bends is critical for optimizing your PCB layouts, especially in high-speed applications like DDR4, PCIe, or RF designs.

Our latest article, Understanding 90-Degree Bends in PCB Design: A Practical Guide, dives into the science behind this debated topic.

It uncovers the real factors affecting signal performance and provides actionable strategies to ensure your designs meet stringent requirements.

Get paranoid about hidden parasitic effects.

Return path parasitics cause voltage drops in your circuits.

These drops generate common-mode currents across conductors.

Cables connected to the Return and Reference Plane (RRP) act as antennas and radiate noise.

Even microamps can lead to EMC test failures.

I see EMC tests as the final exam for your board design.

If the signal integrity aspect is well taken care of, your chance of failing EMC tests is super low.

Why is that?

Because the energy designated for the signal is then efficiently channeled in your circuits, and is used for either information or power, as meant by design.

If these aspects of the design are not well taken care of, then the energy meant for the signals (or power) is wasted in the form of electromagnetic interference and your design suddenly fails the tests, and you can't seem to figure out why.

Yes—it can be that simple to create EMI issues.

Any metal structure in your PCB can start behaving like an antenna if not designed properly.

One common example? Copper fills or copper pours.

If these aren’t handled with care—especially when tied to wires or nets like GND—they can easily become unintentional radiators or receivers of electromagnetic interference.

It’s a small design detail that can have a big impact.

Sometimes it is because of simple details that are not addressed.

For instance, parasitic inductance creates voltage drops in return paths.

These drops drive common-mode currents across conductors.

Cables radiate these currents, acting as antennas.

Just microamps can cause EMC test failures.