Electrical – Design high pass filter

active-filterhigh pass filteroperational-amplifier

I need to create a high pass filter to attenuate frequencies below 50KHz, I intend to use an RC circuit with an operational amplifier.

My question is: How do I set my cutoff frequency? I know that if I set a cutoff frequency of exactly 50K Hz I will get losses in my signal, how do I find the ideal cutoff frequency? Do I Use the Bode Diagram? If so, how does it apply in this situation?

Thanks!!

Best Answer

Here's a bode plot of a simple RC high pass filter's amplitude response - it contains the formula you need to work out the amplitude at every frequency. Use the formula to decide what cut-off frequency you need and whether the attenuation below 50 kHz is suitable: -

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If the filter doesn't attenuate quickly enough below 50 kHz then you need to consider a 2nd order filter or, maybe one that is of a much higher order.

Here's a 2nd order high-pass-filter's response at a cut-off of 48.2 kHz and note that it can produce a peak (resonance) if the Q is too high: -

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I've chosen a value of resistance such that the Q is unity - notice the little peak in the response - if R is smaller the peak rises. The benefit of using a 2nd order filter is that instead of a 6 dB per octave fall in gain at low frequencies, the rate is 12 dB per octave (40 dB per decade).

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