Electronic – Ferrite bead placement

ferriteferrite-bead

I have a question regarding ferrite beads. I have this kind of circuit:

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Where IC1 and IC2 are digital ICs. The voltage supply for the opamps is obviously analog. I want to place a ferrite bead between the analog and digital side. I heard that it is best to place a ferrite bead between two decoupling caps (B). But in a lot of datasheets they place the ferrite bead between analog and digital supply (A).

  • Should I place the ferrite bead in point A or should I use multiple ferrite beads and place them in B?

Best Answer

It depends on the values in the analog section. If both are small (say, 0.1uF and 1000pF, the bead should be upstream. If one of them is bulk or bulk-ish (say, 4.7uF or 10uF) the bead should be downstream of it.

That’s a bit of an oversimplification of course.

This Murata application guide is useful: https://www.murata.com/~/media/webrenewal/support/library/catalog/products/emc/emifil/c39e.ashx

If your digital supply is very noisy, has a lot of ripple, or the analog is very sensitive, you might even consider a separate regulator for the analog part.