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March 8, 2010 / consort3

3rd order low pass filter table

Just to finish the 3rd order theme of the last post here is a Butterworth low pass filter table with 3 equal resistors.

C1 C2 C3 20K 22K 24K 27K
33N 82N 4.7N 340.2 308.9 283.3 252.4
47N 120N 6.8N 236.1 214.4 196.6 175.2
68N 180N 10N 160.8 146 133.9 119.3
100N 270N 15N 107.9 98 89.8 80
150N 390N 22N 73.3 66.5 61 54.3
220N 560N 33N 49.5 45 41.2 36.7

So if you want a filter with 50Hz cutoff look down the 20K column and you see 49.5Hz, the nearest value,  so the capacitors are 220n, 560n and 33n and the associated equal resistors are 20k. The table covers a decade of values. If you want 500Hz as opposed to 50Hz you scale the capacitors by a factor of 10,  so the capacitors would be 22n 56n and 3.3n. Alternatively if you wanted 16Hz the capacitors would be 680n, 1.8u and 100n

Here is a Bessel filter table, the frequencies are the -3dB points.

C1 C2 C3 20k 22k 24k 27k
22n 33n 5.6n 366 330 303 269
33n 47n 8.2n 242 223 204 179
47n 68n 12n 168 150 142 125
68n 100n 18n 115 104 95 85
100n 150n 27n 77 70 65 57
150n 220n 39n 52 47 44 39

PCB’s to produce 3rd order crossover filters available here:

http://www.kk-pcb.com/cross.html

Never mind the old TL074  try the MC33079 quad op-amp for this

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