PCB Ground Plane Design in High Performance Boards
I’ve encountered some interesting surprises while renovating my 1950’s farmhouse. When we moved in, we soon located a ground fault that periodically tripped a breaker on multiple appliances and the adjacent bedroom. Apparently, the electrician that installed the original wiring was an amateur. We got it fixed, but the experience reminded me how important good grounding is in electronics.
Grounding techniques in printed circuit boards affect more than the ability to complete a circuit. Today's high-performance circuit boards run at higher speeds, process more data, are getting smaller, and communicate with other devices wirelessly. They require more layers and denser routing than in the past, but the right PCB ground plane design and grounding strategy will help ensure your board runs as intended.
It Starts With the Right Multilayer PCB Stackup
PCB ground plane design revolves around choosing the shape of the ground plane and placing it in the appropriate layer in your PCB stackup. Your PCB ground plane design starts with the right multilayer stackup. Today's modern digital boards are operating, at minimum, with 4-layer stackups. The image below shows the typical multilayer PCB stackup used in 4-layer PCBs. The ground plane is set on an internal layer adjacent to a power layer. The top and bottom layers are signal layers to allow plenty of room for components and routing.
Higher layer counts might use multiple ground and power plane arrangements, making the stackup more complicated. However, whether we have a 4-layer stackup or more layers, the primary power planes should have a ground plane in an adjacent layer. The two planes will act like a big capacitor that helps ensure stable DC power delivery to your components.
This is just the start of PCB ground plane design. We now need to worry about some specific points with the shape and span of the ground plane. These are some common questions that arise in PCB ground plane design, and these questions extend up to stackups with any number of ground planes. Let's dig into each of these points to learn more about PB ground plane design.
How Big Should the PCB Ground Plane Be?
The PCB ground plane can extend throughout its layer in order to properly do its job, but this is not a strict requirement. In some boards, you might even see a ground plane cut up into sections to provide multiple reference potentials in the design. Technically, you can make the ground layer into whatever shape you want, but if you want to take advantage of a large plane, then you shouldn't start cutting up your ground plane. The ground plane should extend out to the edge of the board and should overlap with the power plane to provide strong decoupling.
There is a particular EMI myth that states the power plane should be pulled back from the edge of the ground plane. This is the so-called "20H" rule, meaning the power plane edge should be pulled inward from the ground plane edge by 20x the distance between the planes. There is no evidence that this produces any EMI benefit. It's best to just span the ground plane throughout its layer to provide strong decoupling and shielding.
What About RF Devices?
In PCB transmission lines carrying RF signals, placing a ground plane below the signal layer may not fully suppress currents from being induced in different circuits. It's very important that analog signals do not experience crosstalk as induced noise will interfere with analog signals, and any digital readout of these signals with an ADC will give an incorrect representation of the signal. This means we need more than just a ground plane below the RF signal layer to provide isolation.
Instead, these signals should be routed as coplanar waveguides to provide much higher EMI suppression. Via fences should be placed in the copper pour surrounding the RF traces, and the via fences then need to be shorted to the adjacent ground layer. This shields nearby components from RF interference and shorts any induced currents directly to the ground plane. In this way, the ground plane and the grounded pour around the signal will act like shielding.
Ground planes provides natural EMI shielding
Antennas and Your PCB Ground Plane Design
Since ground planes are large flat conductors that span a large area within a single layer, they provide natural shielding against EMI. In devices using RF antennas for communication, this can actually work against you as the ground plane can block the RF signal emitted from the antenna. Depending on the type of antenna (whether a printed antenna or an antenna module), You may need to place the antenna above a ground plane.
For a quarter-wave antenna, you need a ground plane to act as an image plane. For a half-wave antenna, you don't need the ground plane because the antenna already generates the negative polarity portion of the emitted field (e.g., dipole antenna). Generally, trace and chip antennas are not be placed above a PCB ground plane.
Instead, place the antenna at the edge of the circuit board and let the ground plane run close to the edge of the antenna. While a monopole antenna is different, in a dipole antenna, for example, the radiation pattern will have a zero intensity line that runs parallel to the length of the antenna (see below). The emitted radiation pattern will be orthogonal to the ground plane, and emission at wider angles will not be blocked by the ground conductor.
Multiple Ground Planes or Ground Plane Regions?
This question arises in mixed-signal systems, where digital and analog sections are placed together on the same board. Proper grounding techniques in mixed-signal systems should aim to separate digital signals from analog signals so that the components do not interfere with each other. The typical strategy is to partition the PCB ground plane into digital and analog sections, meaning to physically split the ground plane into two sections.
The idea is to prevent digital signals from interfering with analog signals. However, there are multiple reasons the ground plane should not be physically split into different sections:
- Difficulty defining clear return paths, which creates inconsistent impedance
- EMI from long return paths with large loop inductance
- Inconsistent reference potential, which creates unintended level shifts
If you do need to use multiple ground planes regions to ensure isolation, you can tie the regions together with capacitors to ensure consistent reference potential throughout the system. However, for high speed/high frequency boards, it is highly recommended to use a continuous ground plane with no splits in each ground layer. This eliminates the possibility of routing over splits or creating wide return paths with high loop inductance.
PCB Ground Below a Clock
If you are sourcing a clock signal in your PCB, some designers will contend that it is appropriate to extend the ground plane beneath your clock IC. This is actually a bad idea, as placing the ground plane beneath the clock creates a center-fed patch antenna. If your clock frequency matches a cavity resonance created by the ground plane, your clock will induce a strong current in the ground plane. This can create additional EMI that interferes with nearby signals, and vice versa if external EMI propagates to the region below the clock.
Since proper grounding is paramount to ensuring reliability, you need a PCB design software package that gives you the best design and analysis tools. PCB ground plane design is easy with the advanced CAD tools in Altium Designer®. If you are interested in learning more, talk to an Altium Designer expert today.
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