Why Ferrite Bead in Supply line

[Dinu Paulson]

Presently Am try to design a USB to UART,485& 232 board, in that Am using FTDI USB to UART converter, In the FTDI data sheet they providing some sample design and A ferrite bead is connected in series with the USB power supply to reduce EMI noise from the FT232R and
associated circuitry being radiated down the USB cable to the USB host. The value of the Ferrite Bead depends on the total current drawn by the application. A suitable range of Ferrite Beads is available from Steward (www.steward.com[/URL]), for example Steward Part # MI0805K400R-10,, So guys could you please tell me How we choose the right part number, my apllication will draw maximum of 30mAs

 

[berkeman]

Presently Am try to design a USB to UART,485& 232 board, in that Am using FTDI USB to UART converter, In the FTDI data sheet they providing some sample design and A ferrite bead is connected in series with the USB power supply to reduce EMI noise from the FT232R and
associated circuitry being radiated down the USB cable to the USB host. The value of the Ferrite Bead depends on the total current drawn by the application. A suitable range of Ferrite Beads is available from Steward (www.steward.com[/URL]), for example Steward Part # MI0805K400R-10,, So guys could you please tell me How we choose the right part number, my apllication will draw maximum of 30mAs[/QUOTE]

The part number you list does seem appropriate for the application. This page will help you understand the various parts of that part number:

[PLAIN]http://lairdtech.thomasnet.com/Asset/Part-Number-Nomenclature-FErrite-EMI.pdf

(Steward appears to have been absorbed by Lairdtech)

In general when you choose ferrite beads for RF impedance insertion, you will first choose the bead material (the ferrite) to match the frequency range where you want a good impedance. With some vendors, materials like #43 are a good match for mid-frequency RF (100-300MHz). Whatever material is being used in this bead is reasonable in its impedance versus frequency plots, although it does appear weighted to higher frequencies (500-900MHz), which may be less of a problem for you in your application.

After the choice of ferrite material, you then choose a bead based on how much "resistance" you want in your frequency range, and choose one that is physically big enough not to saturate with your DC current. Pick a bead that has a rated current value that is at least 2x your application current.