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I'm trying to design my own pcb for the stm32f411 and when i saw the datasheet of the stm32f411 ( http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00115249.pdf ) and then the schematics for the stm32f411 nucleo board i was a little confused.

In pin 13 (Vref+), in the schematics there is an ferrite bead.I can't understand why it used in the Vref+ pin (since there isn't any mention to it in the datasheet not even the explanation of the Vref+ pin function.

Image from schematics of ST - Vref pin and inductor Image is from ST schematics

So, my question is, for what the inductor is used in this application and is it really necessary?

Grateful

Edited

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  • \$\begingroup\$ Note that its not an inductor as in a coil or similar, but a ferrite bead. \$\endgroup\$ – PlasmaHH Feb 17 '15 at 23:20
  • \$\begingroup\$ Could you explain why a ferrite bead is being used in this application? Grateful \$\endgroup\$ – luis dias Feb 18 '15 at 0:09
  • \$\begingroup\$ Beads stop current inrush when power is connected and suppress HF noise. My guess is that it is to remove power supply noise from the analogue voltage reference. \$\endgroup\$ – geometrikal Feb 18 '15 at 0:16
  • \$\begingroup\$ Ok, grateful for your answer. So, do you think it is mandatory to have the bead there? Removing it would cause any problem to the device (this is the first time i see ferrite beads and never worked it them). \$\endgroup\$ – luis dias Feb 18 '15 at 0:21
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That bead seems unconnected by default, through solder bridge 57, which I suspect is normally open. If 57 is closed, it connects Vdd to AVdd, but prevents noise from Vdd hitting the analog ref by increasing high freq impedence.

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  • \$\begingroup\$ That is what i don't undertstand. SB57 is closed, so there is an 0 resistor there. Supposely AVdd is the same as Vdd (Vdd=Avdd), so why there is an bead there? Grateful \$\endgroup\$ – luis dias Feb 18 '15 at 17:50
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    \$\begingroup\$ If SB57 is closed, then AVdd is connected to Vdd through the bead. Think of the bead as a short circuit at DC, with the impedance going up as frequency goes up. AVdd is thus a low-pass filtered version of Vdd, which is good in case there is noise on Vdd. \$\endgroup\$ – Scott Seidman Feb 18 '15 at 17:57
  • \$\begingroup\$ Ah, ok, grateful \$\endgroup\$ – luis dias Feb 18 '15 at 18:45
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Noise on the analog reference means noise on the adc converted signal. So, for a good adc conversion (ENOB = effective number of bits), the noise on the analog reference is crucial. I am pretty sure the bead is to reduce noise. It is a common method.

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