Whenever I find a new artist or album that I love, I tend to listen to them obsessively. I’ll put their songs on loop until even the slightest hint to the beats in their songs will have me humming the whole first chorus. Fortunately, I have friends and family who get sick of hearing the same songs over and over again to break me from this loop. But what if I didn’t want my loop to get broken?
Repetition and loops can have harmful consequences based on the environment that you’re in. This simple lesson was taught to me by both my sister-in-law, forcing me to either mute the album or change it, and by ground loops in my circuits repeatedly causing interference from return path differentials and noise interference. Here, I’ll share what is a ground loop and how to work around some of the vulnerabilities it provokes in your designs.
So, What is a Ground Loop?
In typical PCB design, ground connections provide a return path to the various signals on board. It is often assumed that ground has the same voltage potential on every part of the PCB, but this is often a misconception that is laid bare when there are ground loops in the design.
A ground loop is defined by the existence of a conductive path that is formed by multiple ground connections. In a perfect layout, your components will be connected together properly through ground connections. With only the directly-specified ground connections, components will have a direct current return path, but when a middle ground connection is joined between multiple components, a ground loop is formed.
How Ground Loop Affects Your PCB
In the ideal world, you wouldn’t have to worry about ground loops, because each point on the ground connections will be at the same potential. Unfortunately, the real world of PCB design is far from ideal. When ground loops are present in your design, you’ll find that return current starts finding unexpected paths on the PCB.
Ground loops are a potential cause of noise interference on the PCB. If one component on the loop is noisy and the return current flows through shared ground connectors, this noisy component is then the source for an introduction of noise to other components on the loop. This can be catastrophic with different analog or audio components along the same loop as a particularly noisy component.
You might also think that as long as there are no noisy components on the PCB there are no risks from the ground loops. Unfortunately, you’ll be in for a nasty surprise. A ground loop can also form an inductive loop that could pick up electromagnetic interference from external devices.
Expect both internal and externally induced noise with ground loops.
How to Minimize Ground Loop Effects on Your PCB
There’s practically nothing you can do when your PCB is assembled and suffered interference due to ground loops. If there’s any preventive measure that could have been taken, it would be during the routing process. Working through design and analysis from the beginning phases of your schematic design with SPICE utilities can be particularly helpful in managing expected impedance and noise.
Theoretically, eliminating ground loop means ensuring each component has a single direct path to ground. In actual practice, it is hard to be sure that there are no interconnecting loops on the ground nodes. But you could minimize the chance of ground loops by doing the following:
1. Use a Single Ground Plane
Instead of having multiple ground connections, you should pour a single ground polygon on the PCB. It minimizes the chance of having ground loops.
2. Avoid Splitting Ground Planes
When you have a ground plane poured on the outer layers where hundreds of components and traces are residing, you risk splitting the ground planes. This could also lead to the formation of ground loops. Instead, you should consider moving the ground plane to the internal layers to have a more consistent return path.
3. Minimize Ground Pin to Plane Connections
Having a ground plane is not a blanket guarantee to solve ground loop problems. You’ll want to ensure that components on the PCB are connected to the ground plane with minimal copper trace. This can be done by having the ground connection routed through a via placed close to the component.
4. SPICE Simulation to Reduce Ground Loop Effect
Simulation can trivialize your calculation process by enabling your designs to run through accurate models. By modeling the return conductor in an equivalent electrical circuit by its partial inductance, accurate ground loop coupling can be estimated. Furthermore, SPICE simulators will provide validation for their ground loop estimations by comparing ground current distribution with what would be found in a method-of-moments full-wave analysis solution.
Use ground plane and vias to prevent ground loops.
Thankfully, despite any potential problems a ground loop may cause circuit designs without proper checking or design guidelines being followed, there will always be the opportunity to catch them before you bring the board to assembly.
With Cadence’s strong suite of design and analysis tools, you’ll be able to plan accordingly to work through any potential design vulnerabilities throughout your schematic, simulation, and layout phases. With OrCAD’s PSpice integration, you might even find yourself working through design vulnerabilities you hadn’t even begun to account for by simulating your circuit.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.