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In a lot of circuits, I've seen ferrite beads on the Vdd lines to microcontrollers. For my high speed dsPIC33F (80 MHz, 40 MIPS) microcontroller, should I have ferrite beads on the Vdd lines or should I not bother? The datasheet doesn't suggest using them. I'd like to limit EMI/RF interference, as the module will be used on a model plane and this type of interference could cause problems for the onboard radios.

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To filter high frequency noise. Inductors' windings are capacitive at high frequency so they are effectively useless. If you're worried about your circuit affecting other circuits (or being affected by them), I would only filter the I/O and power entry to your module, so that conducted noise doesn't leave your module on the I/O and power lines, which can act like antennas and radiate the noise, or pick up noise from other modules. The other use, inside a module, is for sharing a voltage rail with sensitive analog components, such as an ADC with a micro. In the case of your PIC, it usually doesn't need such a thing.

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    \$\begingroup\$ it can help you pass FCC cert to use it on places like pics, you want to use caps to filter first, as a ferrite does not do well against high power transients, it operates on absorbing low power transients. \$\endgroup\$
    – Kortuk
    Dec 25, 2010 at 17:55
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    \$\begingroup\$ Ferrites != Inductors : Ferrites are not inductors! Ferrites generally have very few turns, so interwinding capacitance is not important. Also, ferrites are dissipative, rather then inductive, so high frequency energy passing through a ferrite bead gets converted to heat, rather then simply reflected back. This is why they are commonly used for high frequency applications. \$\endgroup\$ Dec 26, 2010 at 5:34
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    \$\begingroup\$ Furthermore, if your dsPIC has separate digital and analog power connections, a ferrite between the digital and analog connections (With caps on both sides) is likely a good idea. \$\endgroup\$ Dec 26, 2010 at 5:39

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