Stay Grounded: Digital, Analog, and Earth Ground in PCB Layout

October 11, 2018 Altium Designer

Retro technology circuitry and grounding

 

In electronics, I get equally confused by the different types of grounds involved in PCB design: digital, analog, and earth ground. It’s hard not to be perplexed when university lecturers have always pointed out that although the grounds share the same reference, they are represented by different symbols.

 

Theoretically, it’s easy to understand that digital electronics components are referenced to the digital ground and the same applies to analog circuits. They are clearly differentiated by the ground symbols used in schematics. However, the real challenge is physically routing and placing the respective grounds on the PCB.

What Does the Ground Actually Do?

Digital currents, whether single-point ground current flow or proper grounding and grounding scheme with reference points, ground pins and ground testers for measurement for digital circuits. Whether you’re trying to incorporate components like an anti-aliasing filter or differential amplifier, or an analog-to-digital convertor, or being wary of signal and noise distinctions in your device or power supply with signal-to-noise ratio and voltage drop, you’ll need to know what you’re looking for.  

 

Sometimes, you need to get back to the basics to master the art of grounding in PCB design. This means going beyond the schematic symbols of the ground. In a typical electronics system, a ground provides a common reference to the signals, both digital and analog. Therefore, to achieve a reliable signal, the ground needs to be relatively ‘clean’.

 

The ground also functions as the return path to the electric current that flows through the signal traces. With respect to the current path, there are two things that you need to keep in mind:

  1. Electric current always travels through the path of least resistance.
  2. The size of the ground trace must be sufficient to handle any high current.

Digital, Analog, and Earth Ground

Digital circuitry is often characterized by the quick transition of signals swinging between the Vcc and 0V. Naturally, the ground would be subjected to spikes, as electric current pulses through. On the other hand, analog signals heavily depend on a stable ground to provide an accurate reading.

 

The earth ground actually refers to the physical connection that extends to the earth itself. It is usually connected to the chassis or used to safely channel electrostatic discharge from adjacent components. Theoretically, the earth ground is a neutral electrical plane; reality, however, proves that it can sometimes be rather noisy.

Connecting Different Grounds on the PCB

Getting the ground patterns correct on the PCB can be a tricky subject, even for seasoned engineers. For instance, a single mistake in audio and digital grounding can lead to static noise coupling to the audio output. It’s common for new designers to use a single ground plane, resulting in the analog inputs fluctuating randomly.

 

Some electronics experts point out that a star connection is one of the best methods to connect the various types of grounds at a single point. However, in some situations, the star connection may not be practical, due to the component arrangement on the PCB or space constraints.

 

After making many mistakes with ground connections, I finally learned how to design stable grounds. I prefer using the ground bar method and segregating the power, digital and analog modules on the PCB. Below is a general diagram of what this grounding system would look like.

 

Power, digital, and analog ground

Sample layout of grounding

 

Of course, this method of grounding is only applicable when components can be clearly separated according to their functions. The principle in arranging the ground planes is placing the noisiest ground at one end and the most sensitive ground on the other end.

 

In some cases, the ground plane only needs to be separated at a specific component. This can be done by separating the ground planes beneath the component in the following manner:

 

Separate ground planes beneath component

Ground planes underneath a component

 

Ground placement in a PCB should not be treated as an exclusive subject. Instead, designers should apply common sense, or rather the basic laws of electronics and a vivid imagination to visualize how electrons travel along the path formed by different ground arrangements.

 

Once you’ve determined your grounding strategies, the actual implementation is pretty easy with intuitive polygon tools in Altium Designer. If you’re unsure about your ground placement or need a refresher on key grounding topics, talk to an expert at Altium.

 

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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