A low-inductance ground plane is essential in PCB design for reducing electromagnetic interference (EMI) and mitigating signal integrity issues.

A larger ground area on the PCB reduces the system’s ground inductance, which in turn helps decrease EMI emissions and crosstalk.

While there are various methods to connect signals to the ground, it’s important to understand what should not be done.

• Never connect PCB components randomly to ground points.
• Use a Full Ground Plane and Ground Grids
• Minimize Return Path Lengths

Faraday’s law dictates that the return current should follow the path of lowest impedance.

Impedance at any given frequency is described by the formula Z(w) = R + jωL.

At low frequencies, the impedance is primarily dominated by resistance (R), while at higher frequencies, the inductive component (jωL) becomes more significant.

This causes the return current to favor the path with the lowest loop inductance.

To optimize signal integrity, it is essential to:

• Keep return paths as short as possible.
• Minimize the loop area to reduce inductance.
• Ensure precise management of the return current paths.

Control Impedance

When designing high-speed circuits, it’s crucial to match the impedance between your signal source and destination.

If this impedance isn’t properly controlled, you can run into issues like signal reflection and high-frequency ringing, which can disrupt your signal integrity.

But the problems don’t stop there.

That excess RF energy from these reflections and ringing can bleed into other parts of your circuit, causing electromagnetic interference (EMI).

This is something you definitely want to avoid. So, how do you address this? The solution lies in signal termination.

By using signal termination techniques, you can reduce those unwanted reflections and ringing.

These methods help control the impedance and even slow down those fast-rising and falling signal edges, making your circuit run more smoothly.

Decouple Power and Ground

When ICs are running, they generate high-frequency switching currents that introduce noise into the power rails or traces connected to them. If this noise isn’t properly managed, it can lead to radiated emissions and EMI issues.

A key way to control power rail noise is by placing decoupling capacitors as close as possible to the IC’s power pins and grounding them directly to the ground plane. Using power planes instead of power traces also helps to minimize power noise and improve overall circuit stability.

Minimize Trace Lengths

Traces on a PCB serve as pathways that carry current from the driver to the receiver.

However, bends or intersections in these traces can create radiating antennas that interfere with signals. Here are some key trace design practices:

Trace Separation:

Maintain sufficient distance between signal traces (such as clock, video, audio, reset signals) by following the “3W rule”—spacing them at three times the width of each trace.

This minimizes crosstalk and coupling between traces on the same PCB layer, with differential traces as the exception. For more on reducing crosstalk, see our guide on HDI substrates.

Routing Differential Traces Closely:

Placing differential traces near each other enhances their coupling factor and maintains noise in common mode.

For instance, if two close traces pick up the same amount of external noise, it will affect both equally, allowing the noise to cancel out.

This is why high-speed signals are often routed as differential pairs.

Smart Use of Vias:

In multi-layer PCBs, vias enable signal routing between layers, but they also introduce capacitance and inductance.

Avoiding vias for critical traces when possible helps prevent impedance mismatches that can cause reflections.

When vias are necessary, add ground vias nearby to stabilize the characteristic impedance.

For differential pairs, ensure an equal number of vias in each trace, and consider using a shared anti-pad to reduce parasitic capacitance.

Avoiding Stubs on Sensitive Traces:

Stubs on high-frequency traces create reflections and can act as unwanted antennas.

Minimizing stubs is crucial for maintaining signal integrity on sensitive traces.