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I'm trying to design a custom board that will include the FGPMMOPA6H GPS.

Looking at the Adafruit integration of this GPS, I can see a ferrite bead on the output of the LDO (MIC5225-3.3). See below:

Schematic

I agree with the use of an LDO to get a clean power supply for the GPS, but I don't know how to choose a "good" ferrite bead on its output.

I'm not very familiar with the use of ferrite beads, so a little help would be highly appreciated!

SamGibson
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p-lafond
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  • Welcome to EE.SE! What ferrite bead is listed in the BOM? – winny May 14 '20 at 09:25
  • Hi, thank you ! :-) I couldn't find any BOM related to the Adafruit integration. Some parts are explicity shown on the schematic (like the LDO) but not the ferrite bead... – p-lafond May 14 '20 at 09:34
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    You won't need the ferrite bead if you find a LDO with low enough ripple. The requirement for FGPMMOPA6H is max 25mVpp according to the datasheet. – Peter Karlsen May 14 '20 at 09:36
  • Yes you're right ! (50 mVpp max though, no ?) But is there a good way to determine how to pick a ferrite bead ? – p-lafond May 14 '20 at 09:45
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    There isn't a good way to determine the FB requirements for the circuit in your question because there is no specification for what the ferrite bead is intended to do. [This post might help understanding](https://electronics.stackexchange.com/questions/221626/how-to-select-appropriate-ferrite-beads-for-power-supply-rails/221640#221640). Also [this one](https://electronics.stackexchange.com/questions/115021/ferrite-bead-position/115113#115113). – Andy aka May 14 '20 at 09:59
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    @PeterKarlsen Beads will not help with ripple, even a bulky Murata BNX025H has its first zero at 2MHz. They are only really useful for switching noise and EMI, so I would place one between PSU and LDO on the VIN-side. – Andreas May 14 '20 at 10:00
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    @Andreas You are right. I woudn't use a ferrite bead at all. The ferrite bead probably originates from the FGPMMOPA6H reference design in the datasheet, where the source of supply is unknown. It is most likely just copy/pasted by adafruit. The bead serves no purpose where it's placed. It would be better to put it on VIN of remove it completely. – Peter Karlsen May 14 '20 at 10:16
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    A low drop out (linear) regulator doesn't produce ripple itself. A ferrite bead seems out of place in that circuit. – JRE May 14 '20 at 11:55

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