Determine output filter using the bode approximation
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There is a slope of -20 dB/decade per each pole. So you calculate how many decades there are between Fc and the frequency in question (x = log10(f/Fc)), and you add as my poles as needed. Note that if you place the pole at Fc you will add a 3dB drop at Fc. So you will need a 38dB attenuation on your interferent frequency instead of 35dB.