EMC Question of the Week: November 18, 2019
A ferrite bead should be placed in series with the ground connection
- on analog-to-digital converters
- on stable voltage references
- when the app note specifies it
- all of the above
- none of the above
Answer
The best answer is "e". There's really no excuse for placing a ferrite bead in series with any trace or connection labeled "ground" or "gnd". In fact, these beads are likely to make high-frequency noise issues worse, while having little effect at all at the low frequencies where isolated current returns might be required. Connections to ground on analog-to-digital converters should generally go directly to the circuit board's ground plane. The same applies to stable voltage references and clock multiplier circuits. The power supply connections to these devices often require filtering, but the power return (often labeled ground) should not have a series impedance.
While this is generally understood among EMC and signal integrity professionals, there are many application notes (particularly older ones) that still recommend putting a ferrite on the ground. This is particularly bad advice that has caused significant noise related problems in some products.
Generally speaking, application notes are a poor source of information when it comes to circuit board layout for meeting EMC requirements. Yes, some application notes provide very good guidance; but others rely on out-of-date design guidelines or bad models to recommend layouts that are truly awful. If an application note was published more than 10 years ago, or if it makes recommendations that seem contrary to generally accepted practices, it should not be trusted.
Have a comment or question regarding this solution? We'd like to hear from you. Email us at