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Active filter design : 2nd order sharp roll-off

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grouchobyte:
Try this. Your requirements are a little fuzzy,but based on comments you made I took a stab at it. Is this what you want/need? If not, please clarify with better requirements like ( for example):

high pass?
nth order?
fidelity in the time domain/ dont care?
input/output impedance?
pass band attenuation?
pass band ripple?
topology preferred ( active/ passive, sw cap...etc)
rolloff?


Let us know and we can help you optimize it

grouchobyte:
your want better rolloff?.....try this. Sim attached

You're welcome!

@grouchobyte

nick_d:
The easiest way to meet this requirement is to put the correct number of Sallen & Key high pass filters in series. Each Sallen & Key filter uses one op-amp and two RC networks to deliver (in voltage) 6dB/octave = 20dB/decade or (in power) 12dB/octave = 40dB/decade of roll on. See here: http://sim.okawa-denshi.jp/en/OPseikiHikeisan.htm

The choice of response type (Butterworth, Bessel, Chebyshev etc) is independent of the number of sections. What these options do is change the behaviour slightly near the cutoff. A few octaves away from the cutoff the behaviour will be basically the same for all response types. If you have say 3 sections with 6 RC filters, then you set the response type by tweaking the 6 RC values so that each has a slightly different cutoff, using a table that is pre-computed for cascaded Sallen & Key filters of a particular number of sections.

There isn't really anything more to do. It is a completely straightforward exercise in plonking down the example circuit and reading the tables.

cheers, Nick

edit: clarify voltage dB vs power dB

Rutherberg:
Thanks a lot for all the answers despite the lack of clarity of my request.

Attached, 2 circuits simulated, green curve is a high-pass Sallen Key calculated with okawa-denshi calculator, blue curve is a 1-stage Chebychev calculated with Analog filter wizard, cut-off is approximatively 3,7kHz for both.

The rough red curve is what I need : at least the roll-off of the chebychev or sharper at approx. 3.7kHz, but limit the attenuation.
I would like to have 1kHz attenuation at about -14dB.
Filter ripple is not crucial, but better under 2dB.
Also, I would like to keep the numbers of stages to a minimum, ideally 1 or max. 2.

How can I limit the attenuation ?


--- Quote from: grouchobyte on February 05, 2019, 10:23:10 pm ---high pass?
nth order?
fidelity in the time domain/ dont care?
input/output impedance?
pass band attenuation?
pass band ripple?
topology preferred ( active/ passive, sw cap...etc)
rolloff?

--- End quote ---

- High pass
- 2nd order
- Don't care
- Don't care, will be buffered
- minimum possible
- max. 2dB
- Don't care, but this circuit will be audio and thus noise sensitive
- faster/sharper than the green curve attached

Is it more clear ?

Thanks

nick_d:
Are you happy that from 0 to 1kHz you have exactly 14dB attenuation, and then after that you have the Chebyshev? Since your circuit will be buffered, that is easy to do. 14dB is a factor of 25 (calculate by 10^1.4) if you meant by voltage, or 5 (calculate by sqrt 25) if you meant by power. So you create the Chebyshev filter and then present your buffer with a voltage that is 1/25 the original signal plus 24/25 the filtered signal. You do this with a resistive divider. Like this:

orig
  |
  |
 [ ] 24k
  |
  *--- out
  |
 [ ] 1k
  |
  |
filt

The voltage at "out" will be the weighted average of "orig" and "filt", however make sure your buffer doesn't take much current from it or it will reduce the accuracy. The best way to do it is have "out" be either the + input of an op amp in voltage follower configuration, or the virtual Earth of an op amp in inverting configuration. This ensures an accurate weighted average or sum respectively.

cheers, Nick

Edit: are you sure your Chebyshev is one stage? A Chebyshev has at least 2 RC filters, since what makes it a Chebyshev is the adjustment of the two RC cutoffs relative to each other. I think your blue and green lines have same roll-on but different response types. May be wrong but possibly 1 stage means two RCs?

Edit: swap the 1k and the 24k.

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