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Ferrite beads VS resistors

Luca , 02-17-2017, 02:48 PM
Hi to every one, it's possible to have some trouble if i put a small resistor, like 5 to 20 ohm for example, in series with ferrite beads that supply the analog section of microcontroller?
I would like to add for simply measure the voltage drop across and then know how much current the analog section of microcontroller needs and also for reduce the Q of the filter.
alazareff , 02-18-2017, 10:45 AM

It depends of the current. For exemple, if you have a 100mA current with a 10 Ohm series resistor, on a 3.3V, then you have a 1V drop => 2.3V on the power supply pin. If the minimum value of your microcontroller is 3.1V, then the microcontroller won't work, or badly.

So, try to find a value which provide enough margin for the microcontroller power supply.

And you can also simply use the ferrite for the measurement, since it has a small known resistance (measure it before). Except if the current is too small.
mairomaster , 02-18-2017, 12:28 PM
I would rather worry about noise. If the analogue section requires as clean supply as possible and it happens to consume considerable none-constant current, that will introduce some noise due to the changing voltage drop around the resistor. Maybe you can use this for development only and remove it or change it to 0R in production.
saeednowroozi , 02-18-2017, 11:23 PM
Hi all
anyone have a reference about how to use ferrite-beads? i see a lot of project use ferrite-beads with variable value for microcontroller and fpga (startan6) but i can't understand for example: 5v need a ferrite-bead with a value but for other part(sata,...) a other ferrite-bead.
Luca , 02-20-2017, 05:52 AM
Thanks at all for the reply!
robertferanec , 02-20-2017, 11:00 AM
@Luca what I have seen, for measuring, the often used value of resistor is 0.1 OHM - but it depends on currents. Also, I would not place any resistor in series with BEAD, I would rather replace the bead with 0.1R, do the measurements and put the BEAD back.

@saeednowroozi, I normally use BEADS based on reference schematic, but we often reduce number of types to two - BEADS used on powers (they have high current rating, but may have slightly lower filtering capabilities), and BEADS for smaller currents (smaller package, may have better filtering capabilities comparing to the high current BEADS). If you are not sure, have a look at some calculators (I do not really use them), they can help you to choose the BEAD .... maybe something like this: http://www.murata.com/en-us/tool/download/emc
saeednowroozi , 02-20-2017, 11:25 PM
Thank you Robert
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