EMC Question of the Week: June 29, 2020
Which of the following represents a proper layout for a pi-filter on a printed circuit board?
Answer
The correct answer is "d". The first three options all have inputs and outputs that share a ground connection. This shared connection impedance allows noise currents on one side to couple to the other side of the filter.
Filters with input and outputs that share a noise current path are never appropriate, yet layouts like those in options a.) to c.) continue to appear in some products. This is often the result of bad advice in app notes and datasheets that confuse the concept of "ground" with the concept of "current return." Single-point grounding is an appropriate grounding strategy in some circumstances. On the other hand, "single-point current return" is never a good option.
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