Ferrite Bead for FT232RL?

Steel

The FTDI Website shows a Ferrite Bead at VCC from the Host.· I am wondering if this is necessary (In order for the host to pass USB specs, they need clean VCC).

If so, does anybody have any recommendations.· "All these beads look the same to me...."

Thanks

Beau Schwabe

I'm not sure what the actual inductance value of the ferrite bead is supposed to be, but a ferrite bead is used to block high-frequency noise from entering the power supply.
In other words chances are that the power supplying the USB_to_Serial adapter is relatively clean, however the chip set that converts the USB to serial can potentially introduce noise
that will make it's way back into the supply line where ALL of the other components powered by the same supply will "see" the noise. By using an appropriate inductor, much of the
noise can be eliminated at the source.

As far as your question.... "if this is necessary" ? .... It will most likely work without the bead, but what's unpredictable, is that you may have OTHER circuits that are powered by the
same supply that may be far more sensitive to the noise on the power supply that can experience problems.


www.murata.com/emc/knowhow/pdfs/te04ea-1/23to25e.pdf
www.ftdichip.com/Documents/Schematics/FT232R/UC232R%20Schematic.pdf

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Steel

Thanks, Beau.