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Basic Electronics » How do I calculate the cutoff frequency...?

January 16, 2011
by lcruz007
lcruz007's Avatar

How do I calculate the cutoff frequency of a low pass filter that has different R and C values? I'm using a 3rd order low pass filter. Do I add every resistor (R) and capacitance value (C)? Then I'd apply the formula?

1/[2pi(R1+R2+R3) * (c1+c2+c3) ]

Would that be right? ...

This is the schematic I'm using: http://img810.imageshack.us/img810/8945/filter.png

What's the cutoff frequency?

Thanks in advance.

January 16, 2011
by bretm
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There isn't just one simple cutoff frequency necessarily. There will be a transfer function that tells you the attenuation (or gain) at each frequency. You can calculate what it would be for a specific frequency by calculating the impedence of each capacitor and replacing them with equivalent resistors and calculating the op-amp gain.

January 17, 2011
by bretm
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I just noticed this schematic looks a little weird. I don't see what R10 would do because there won't be any significant current going through it. It looks like the capacitors all got shifted "one resistor to the left" so to speak. It should look more like this, and that page will also calculate the frequency response for you.

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