Understanding the Causes of Ground Bounce in PCBs

April 9, 2020 , in Blog

Optics is one of the burgeoning areas of engineering that is finding a home in an ever-expanding list of applications. One of the more exciting yet controversial of these is facial recognition, where the question of when and how visual profiles should be used is constantly debated, especially when there are concerns over individual privacy. Other usages that are viewed as benign are various types of mapping, including topography where a two-dimensional image can be transformed into a 3D representation of a surface such as the earth. This type of technology depends upon the ability to accurately distinguish between different heights or levels of the ground.

Topography in two dimensions

2D topological view with varying heights

Different levels of ground can also exist on your PCBA. Although this may not be desirable, it can be a natural result of PCB layout complexity for boards with multiple signal types. With the continued miniaturization of circuit boards and demand for non-standard PCB form factors, routing traces to maximize signal and power integrity can be quite challenging. A major reason for this is the potential for ground bounce, where the potential of a ground may change resulting in different ground levels. Prior to looking at how ground bounce in PCBAs can occur, it is probably informative to first see what ground actually is for circuit boards.

Is Ground Really Ground?

In electrical and electronics engineering, ground is a term that gets tossed about quite a bit. For large transmission and distribution (T&D) networks like the US grid, references to ground invariably mean actual earth ground. For electronics systems, the situation is a bit more complicated. First, it is rare that a PCBA is connected directly to earth ground. However, circuit boards that are installed in structures: residential, commercial and industrial; do have paths to earth ground. Typically, this includes the board ground(s) that are connected to the chassis ground that in turn to connects to earth ground. In this case, we can say that the board ground is really ground or indirectly grounded.

There is a large class of electronic systems that are not connected to earth ground. This includes automobiles, aircraft, and portable devices that are battery-powered. These systems are invariably insulated from contact with the earth by rubber tires or system enclosures, which also prevents current from finding a path through external contact with people, which may cause serious injury or even be fatal. For these systems, the chassis ground serves the purpose of the earth ground. For example, if your car is struck by lightning, most of the current will flow through the steel cage (chassis) and arc to the earth. However, circuit boards and other devices that are or contain metal can experience elevated current flow and suffer damage. In this case, PCBA ground is not really ground, but relative ground.

Why Does Ground Bounce in PCB Layouts Occur?

Whether your PCBA is really grounded or relatively grounded, the ground voltage on the board itself can vary depending upon where the ground connection is made. A major cause of this variance is ground bounce, which can be defined as follows:

Ground bounce on a PCBA is when the voltage level of a ground reference changes value or “bounces” to a different value. This phenomenon is usually associated with gate switching within transistors and ICs, and if significant enough, can cause erroneous circuit operation.

As shown in the figure below, ground bounce can change the ground voltage potential to a level to where digital circuitry can malfunction. For example, interpreting a ‘0’ as a ‘1’ can cause flip-flops to change state and transistors to shut off or turn on.

Oscilloscope measurement of ground bounce

Ground bounce example

(Image source)

Depending upon the PCB layout and number of ground planes, transistors, ICs, and other digital circuitry may be physically grounded at different points. Each of these connections has a series virtual inductance along the trace. These parasitic inductances cause the ground at the component pin to vary from that at the plane connection point and must be considered to mitigate the effects of ground bounce. In addition to disrupting digital circuit operation, ground bounce may be felt as noise throughout your board. Therefore, you should always follow good tips to reduce ground bounce, especially for high-speed boards.

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One of the most fundamental requirements for well-designed PCBAs is to furnish adequate grounding. Meeting this objective extends beyond the number of ground planes of your stack up and for high-speed boards must include consideration of ground bounce causes. As ground bounce is a physical phenomenon, its effects depend upon your board’s construction. At Tempo Automation, we have the expertise and manufacturing capabilities to assist you in successfully minimizing the effects of ground bounce.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on ground bounce in PCB layouts and how to minimize its effects, contact us.

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