In a closed circuit, a return path must be provided for current to flow back to the power source; this return is often referred to as an electrical ground. Ideally, these ground connections would have no resistance or parasitic capacitance, and all ground references could be assumed to be at the same potential. However, all wires have small resistances as well as parasitic capacitance.
Evaluating a laboratory measurement with multiple instruments (and multiple power supplies) compounds this problem. When two or more devices are connected to a common ground through different paths, a ground loop occurs; the voltage differences generate current in the form of induced noise. This ground loop noise can seem to appear or disappear for no obvious reason, which can make diagnosing the noise extremely challenging.
When troubleshooting a circuit, it is best to avoid changing multiple variables at once. The following tips can help you take a more methodical approach.
3 Tips for Avoiding Ground Loops
1. Create a single grounding point.
By creating a single grounding point, usually at the measurement ground, you can avoid the potential for ground loops in the first place. While this may not always be practical to implement, given the physical location of the electrical components, it is a good guiding principle that will help you avoid most problems.
2. Look carefully for unintended ground paths.
I know of a situation where a ground loop was created because the DUT housing was in a cryogenic Dewar, which was resting on the concrete floor, upon which rested a metal table that was supporting an instrument with a grounded housing. This is not a “circuit” in textbook terms, but it functioned like one. The real challenge of diagnosing pesky ground loops is often your ability to think creatively to find the loop.
3. Maintain ground connections.
By resisting the urge to remove all ground connections, you can avoid an increase in noise due to an antenna effect. Keeping the case ground on instruments also provides a safety ground in the event of an internal fault condition, preventing the case from reaching dangerous voltages. Always preserve the safety grounds if you change the grounding scheme on a system.
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