|Jan31-13, 09:32 AM||#1|
Ferrite bead usage
How much is too much, when it comes to using ferrites.
Some of the boards I designed previously did not even have single ferrite. We pretty much never used ferrites. Compliance was not a problem.
Now I design ARM9 based embedded system boards. There are ferrites for everything - NAND, DDR, CPU, CPU core. Are so many ferrites really needed?
worst of all, there is no justification. They continue to use it just because its been used previously or it was used in the reference schematic.
One ferrite after the power supply is great. But for every rail?
It basically creates split planes. So no single reference plane for high speed signals. Makes layout hard, specially when DDR2, NAND are present.
|Jan31-13, 07:50 PM||#2|
EMI is difficult and very much experimental, so what one designer does results primarily from his habits.
Because EMI worries take so long to solve, and a ferrite bead or decoupling capacitor or voltage regulator is cheap, experimented designers use to put more than strictly needed, in accordance to hard-learned systematic practices, to save stress and time - and leave them in place when the design works, instead of trying each and every one.
As a result, typical designs have more filtering elements than needed. Possibly many more.
If you wish, you could try to suppress some and take the very long time to check if the circuit still works perfectly. Remember that a digital circuit's error-free operation is very long to check - at every supply drift and temperature and timing skew.
But don't first design a simplified PCB, run into trouble, and try to improve it later. Take the paranoid design first and remove gradually some elements instead. If not, you'll fail.
And you knwo what? I suggest that you evaluate how much the beads cost and how much time you'd waste (months and several runs of design+cable+test) if it runs wrongly.
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