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Often I have a 3.3V or 5V mixed-signal circuit with a microcontroller running in the 10 MHz range and some analog circuitry, e.g. audio or analog sensor signals, powered from a wall-wart style supply or USB.

So a problem I often come across is how to filter the supply for the analog signals. Shall I use a ferrite bead or a choke? More inductance or less? Or just an RC filter?

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  • \$\begingroup\$ An LC filter will be more effective. \$\endgroup\$ – Leon Heller Jan 26 '11 at 4:15
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For filtering large frequencies I'd recommend the ferrite. The parasitic capacitance of inductors makes them essentially useless for filtering high frequencies.

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  • \$\begingroup\$ What do you mean by "filtering large frequencies"? I was talking about filtering the power supply, that's DC. \$\endgroup\$ – AndreKR Feb 12 '11 at 3:10
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    \$\begingroup\$ You're trying to filter high frequency noise from the micro (10+Mhz), switching regulator and such from the analog supply rails, no? If you have a perfect DC supply with no noise then there is no need to filter. Usually, ferrites (with ceramic capacitors) are used to filter analog rails from digital rails because at high digital frequencies inductors are ineffective. \$\endgroup\$ – bt2 Feb 12 '11 at 5:03
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Typically I find that just adding a larger capacitor is all that is needed. If you place multiple capacitors in parallel of values that decades of each other you can help filter out noise at different frequencies. Ex, 0.01uF, 0.1uF, 1uF, 10uF. Using surface mount components is ideal for minimizing noise.

If you need cleaner power you can use an LC filter. This is far less common to see on boards though.

Another thing you want to look at is a voltage regulator. If you are already using one, check the datasheet to see if you are meeting all of the requirements that are required. These usually include input capacitance, output capacitance, and capacitor ESR. If you aren't using one, look into using one as you will be able to clean up your power a lot.

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  • \$\begingroup\$ It is beyond question that I use a combination of big aluminium and ceramic, staggered by ESR and that I have a voltage regulator with good line regulation and low ripple. But the capacitors will probably do their job better when I provide them with an L or R. I'd like to avoid noise that I don't yet see on the lab bench, eg. from SMPSs in the neighbourhood. \$\endgroup\$ – AndreKR Jan 26 '11 at 14:58
  • \$\begingroup\$ @AndreKR, Why is it beyond question? What is your noise budget/target? \$\endgroup\$ – tyblu Feb 12 '11 at 5:56

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