I'm trying to design an LC low pass filter to attenuate 30-35kHz noise on a 3.3V power rail. It's preferable to attenuate higher frequencies as well. So I designed an LC filter with L=10u, C=1u (which is wrong, but this is just an example) - and I found it boosted frequencies near the cut-off point. Is there any way to avoid this? The boost of 22dB corresponded to an increase in 12.5x the voltage. The rail will be powering a microcontroller, which draws 100mA, so this increase in voltage would likely damage it.
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In simple terms, add a resistor in series with the inductor. But then you could rather use a RC filter because you're decreasing the quality of your inductor.
Why do you prefer and LC filter? The filters are harder because at a certain frequency, you will get a resonance point. The voltage you put on the filter will be 'amplified'. The amplitude of how much it will shoot up , is called the Q factor. To dampen the Q , you add a series resistance to the coil to dampen it out.
A lot of math stored on: http://en.wikipedia.org/wiki/RLC_circuit
But because you're only operating at 30kHz, I think a RC filter might do the job better/easier.
Edit: Sorry your post was confusing because first you were talking about a 3.3V >signal< and then about a power line.
The answer still stays similar, add a small resistor, but it will reduce the steepness of the filter. Alternatively you can better add a filter which it's cutoffpoint is way less than your noise. I.e. if you have 30kHz noise, add a filter with a cutoff point at 3kHz or something. You won't get the increase in amplitude at the noisy frequency and a pretty good attenuation.
However, as mentioned, patching the source is always better.
If you are using LC filter for any voltage (power supply). Don't select the cutoff frequency of filter has same as frequency you want to attenuate. Generally , you should select 10 times less than the frequency you want to reject. Make sure that Inductor SRF is away from the frequency. Inductor current rating should be 2 times of the max current comsumption of load and voltage rating for capacitor should be greater than 3 times of the selected voltage.