EMC Question of the Week: August 17, 2020
The reason some application notes recommend returning two or more signal currents to a single common point is
- to give those signals the same zero-volt reference
- to reduce common-impedance coupling
- to eliminate ground loops
- because app notes are often poor sources of EMC advice
Answer
The best answer is "d". While application notes sometimes include layout advice for addressing EMC concerns, that advice is often wrong. Two common themes of the bad advice in application notes seem to be,
- single-point current returns, and
- isolated high-frequency grounds.
Both of these are related to general confusion regarding the role of Current Return conductors versus the role of Ground.
Single-point grounding (or star grounding) is a useful concept. It allows circuits with isolated current returns to have the same zero volt reference. The purpose of the isolated returns is generally to reduce low-frequency common-impedance coupling. It's important that the ground conductors connected to the single point don't carry signal or power currents.
Single-point current returns can occur when people confuse ground conductors with current return conductors. Routing the return currents from different signals to a common point enhances common-impedance coupling and is generally a terrible idea.
A good description of the roles of both ground and current return can be found in the Grounding tutorial at https://learnemc.com/grounding.
Admittedly, any of the answers above can be justified, because the question asks for the "reason" for the advice, and not whether the reasoning was correct. Nevertheless, it is important to recognize that (from an EMC perspective) there is virtually NEVER a good reason to route isolated signal returns to a common point. This is especially true for high-frequency signals.
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