# Ripple in filter.

Welcome all. I have pretty basic question however cannot figure it out. I have two filters LPF and BPF pictures below show them. How can I calculate ripple in pass-ban and stop band?

Is this a homework assignment?

@ wolframore. Nope. I designed system that has two filters in them however now I want to list and factor in errors and inaccuracies.
@ ReverseEMF ok no problem.

ok for your first example with the opamp… since you’ve failed to post the image, it’s a LPF with the f @

f= 1/(2πRfCf) I’m working with a similar circuit at the moment.

So I know when cut-off frequency starts and roll off factor. What I need to find now is how many dBs I can expect in stop-band.

the cutoff freq is where there is a 3dB roll off... it should roll off about 6dB per octave with each octave being reduced by half. You will need a better filter if you require more roll off.

This really isn't Arduino forum material.

Yes agreed 40 dB per decade for 2nd order. Cut-off at -3dB. This part I have so far however in stop band of low pass filter there is still “ripple”. This part I have no clue how to calculate it.

This part I have so far however in stop band of low pass filter there is still "ripple"

No there is not. There is no ripple in second order filters. At lease not what electronic engineers call ripple.

By ripple do you mean a signal of say 50 or 60 Hz? If so that is mains pickup and can not be calculated as it is a function of the electromagnetic environment you are in, physical layout of the components and lots of other things that are hard or impossible to calculate.

Dear Grupmy Mike agreed! After wild goose chase, I figured that is tricky and irrelevant question. Reading about designs different to mine like elliptic or Chebyshev filters they do have “ripple” as their gain is not straight line in pass band and stop band. However, in case of RC 2nd op-amp based BPF or passive RC LPF I believe it is not an issue.

Ohh that’s what you’re asking. That would be a big issue for audio and rf circuits if these filters added ripples.

wolframore:
Ohh that’s what you’re asking. That would be a big issue for audio and rf circuits if these filters added ripples.

Not really RF circuits, narrowband LC filters (or their equivalents) are used and passband ripple is common.
ADC and DAC signal conditioning is much more relevant, and careful control of ripple and phase errors are important with both for data acquisition systems and the like.

Actually it’s an issue in audio as well. I’m not just talking about adc but analog. High Q filters have issues and used very gingerly for this reason. It can cause all sorts of phase issues