Whether you are professional electrical engineer or part-time DIYer, before you start your next project, read through this primer on grounding. This short survey covers one of the most fundamental topics in electronics: grounding.
Electronics Signal Ground or Circuit Common
Signal ground is the current return to the power supply. Current leaves the power supply, passes through the various electronic components, and then returns to the supply. The typical symbol for signal ground is shown in Figure 1.
Chassis Ground or Earth Ground
Chassis ground is an electrical safety requirement to prevent an electrical or electronic device’s chassis from delivering an electrical shock. A long copper rod is driven into the ground outside of the building, and a wire connects the metal chassis to the rod which is at the approximate 0 V potential of the earth. The symbol for earth ground is shown in Figure 2.
Ground Details
Consider the following two details about ground. First, ground is not exactly 0 V. And second, two physically different ground points will not be at the same voltage potential.
Ground Loop
By definition, current will flow in an electrical conductor connected to a difference in voltage potential between two points. Because two physically different ground points are not at the same potential, current will flow through an electrical conductor connected between those two points. This is a ground loop.
Notice this current flowing between these two different ground points is not related to or correlated to any electronic data or message signal. This is noise or garbage that will interfere and distort any information contained in the electronic system.
Note: While “noise” can be added to systems on occasion, it is specifically controlled and the exact quantity is regulated.
Example
Given: A ground loop producing 610 μV of ground noise. It’s a very small quantity. You have a 16-bit A/D converter with a 0- to 10-V input. The smallest voltage it can resolve is:
= 10 V/16 exp 2
= 10 V/65,536
= 152.5ìV
Note that the ground loop noise is four times greater than the actual data, so that A/D converter loses two bits of resolution, and it is now a 14-bit converter.
Connect with Single-Ended/Unbalanced Amps
In Figure 3 the two grounds exist at different potentials, so some current will flow between the grounds. 
This ground current has nothing to do with any signals being amplified, and it is noise decreasing the accuracy of the system. Figure 4 is a complete schematic.
Connect with Transformers
When connecting with transformers, keep the following in mind:
- There is no ground connection, so there can be no Ground Loop.
- Common-mode rejection of RF interference.
- Signals are AC coupled, so of limited use for circuits with DC data such as accelerator focus and bend magnets (see Figure 5).
Connect with Differential Amps
Refer to Figure 6 for connecting two systems with differential amplifiers.
- There is no ground connection, so there can be no Ground Loop.
- Common-mode rejection of RF interference (see Figure 7).
- Signals are DC coupled, so this is the perfect solution for circuits with DC data.
—Dennis Hoffman
Note: This article first appeared in audioXpress (June 2011). It is from a class that Dennis Hoffman teaches at the SLAC National Accelerator Laboratory (Menlo Park, CA).
Circuit Cellar's editorial team comprises professional engineers, technical editors, and digital media specialists. You can reach the Editorial Department at editorial@circuitcellar.com, @circuitcellar, and facebook.com/circuitcellar
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