Ground Circuit Impedance Tests: Ensure Proper Grounding on a PCB?

July 22, 2019 Cadence PCB Solutions

Cars stuck in traffic heading toward a city

 

There are perks of living in a metropolitan city. The endless choices of dining, entertainment, and shopping outlets are a reason to hustle through the busy lifestyles. The same can’t be said on the traffic though. An oversight on rush hour has turned a 15 minutes drive into a 45 minutes journey. 

I’m particularly obsessed about not getting trapped on the road. And that same obsession extends to PCB design, especially where ground circuits are concerned. Often overlooked by designers, how a ground circuit is designed on the PCB can affect its impedance and may cause signals to take the longer path home. 

What Is Ground Circuit Impedance 

Regardless of the types of design that you’re working on, ground circuits are a common occurrence in each of them. It is easy to take ground circuits for granted. On paper, it is the perfect return path for current that flows out from a source.

When you’re transferring a circuit between component selection and schematic creation, you assume that the signal will take the path as you’ve imagined. However, the actual current path on the PCB may differ, thanks to the impedance of the ground plane or trace. 

It’s a misconception to think of ground of only consisting resistive elements. The copper ground traces on the PCB also consists of inductance and capacitance, which translates into impedance. Ground impedance became prevalent when high-frequency signals are involved. 

 

circuit board in front of smith graph

Ground impedance can affect signal return path.

 

When designing a circuit, it is the goal to have the signal return path to be as close to the originating source to prevent issues like EMI coupling and also for transient protection. Therefore, having a low impedance ground path is important.

Why Ground Circuit Impedance Tests May Not Be Feasible On A PCB

Impedance testing is common for high-speed signal traces. It is performed with the aid of tools like a Time Domain Reflectometer or a Network Analyzer. This begs the question of whether ground circuit impedance tests are possible as it is obvious that the impedance of the ground circuit is important. 

In PCB design, ground circuits are usually designed with copper planes. Unlike signal traces, copper planes may vary in dimensions and cover a large area of the PCB. While it is still possible to obtain the impedance value of the ground plane, it isn’t feasible to do so compared to measuring the impedance of signal traces. 

That’s because in most cases, it is the return path of the signal that you’ll want to be concerned off. A high-frequency signal is supposed to have a return path close to its driver, but when the ground adjacent to its signal has a high impedance, it may stray into the ground meant for other components and introduce ground noise. 

Unless you’re breaking up the ground plane in smaller pieces and measuring the impedance of each, there isn’t much merit in getting the impedance value of the entire ground plane. But that doesn’t mean you should neglect in designing a ground with low impedance.

How To Ensure Proper Grounding Without Ground Circuit Impedance Tests

I can’t stress how important it is to have a low impedance ground. That means having the shortest return path possible for a signal. With ground circuit impedance tests for PCB out of the question, it all depends on some of these best practices.

1. Plan The Ground Return Path

There’s a saying that ‘failing to plan is planning to fail’ and that’s absolutely true when it comes to grounding in PCB. While it’s important to have ground separation for analog and digital signals, it is equally crucial to plan the return path for signal traces, especially high-speed ones. 

It’s always a bad idea to generate a ground copper polygon around the signals and hopes that the current will take the shortest return path. Instead, it is better to allocate ground return path adjacent to or underneath the signal rather than leave the design to chances. 

2. Use Ground Return Vias

Just because you’ve had the ground planes poured on the PCB and the DRC check returns no error doesn’t mean it is all fine. The number of vias interconnecting the ground planes is important, as it may affect the impedance of the return path.

 

Green circuit board with vias in the center

PCB vias can reduce ground impedance.

 

The rule of thumb is to have ground vias placed adjacent to high-speed signals, to ensure a short return path. Also, having multiple ground vias decreases ground impedance, which also prevents the ground loop from occurring in the PCB.

Using the right PCB software can eliminate ground circuit impedance issues. OrCAD PCB Designer enables designers to perform return path analysis and capture potential issues in the circuit stage before it hits the schematic or as you’re working alongside PSpice in OrCAD’s Capture tool. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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