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| Active filter design : 2nd order sharp roll-off |
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| Karel:
https://www.ti.com/design-tools/signal-chain-design/webench-filters.html |
| Rutherberg:
--- Quote from: nick_d on February 06, 2019, 12:04:34 pm ---Are you happy that from 0 to 1kHz you have exactly 14dB attenuation, and then after that you have the Chebyshev? --- End quote --- I misspoke, I would like -14db from pass band to 1kHz, and approx. -20/-22dB from pass band to 100Hz. Just like the red trace on the graph. I tried with 1k and 24k, it limits the attenuation, but attenuation curve is not smooth, I got a dip at about 1kHz. --- Quote from: nick_d on February 06, 2019, 12:04:34 pm ---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? --- End quote --- Yes, by one stage I mean one op amp. --- Quote from: nick_d on February 06, 2019, 12:04:34 pm ---Edit: swap the 1k and the 24k. --- End quote --- Yes, I tried too. |
| Conrad Hoffman:
You can do a nice 4-pole filter with a single opamp. See my program link in this thread- https://www.eevblog.com/forum/projects/third-order-filters-with-a-single-opamp-are-possible-after-all/25/ There can be component sensitivity issues with any higher order filter, but I've had little trouble. |
| nick_d:
--- Quote ---I misspoke, I would like -14db from pass band to 1kHz, and approx. -20/-22dB from pass band to 100Hz. Just like the red trace on the graph. --- End quote --- Can you clarify if you are talking about power or voltage? Assuming voltage, you cannot get 6dB/decade. You can get 10dB/decade which might be adequate. Assuming power, you can get 20dB/decade which is steeper than you need. What you will have to do is construct the response stepwise. So, you can have cutoffs at 100Hz and 1kHz. This gives you 3 signals to work with: original (freq 0..infinity), first filtered (freq 100..infinity) and second filtered (freq 1k..infinity). A weighted average of these can give you the stepwise response that you require. If yoy want the steps to be as gradual as possible, use one RC. If you want the first step gradual and the second sharper, use one RC for the first filter and the two-RC Sallen-Key for the second. You can design what you need. --- Quote ---I tried with 1k and 24k, it limits the attenuation, but attenuation curve is not smooth, I got a dip at about 1kHz. --- End quote --- That shouldn't happen. Can you show your circuit? You probably loaded down the resistive divider by taking current from it. Use one of the techniques I mentioned (mean and voltage follower, or summing junction). More sophisticated responses can be constructed with digital techniques. cheers, Nick |
| grouchobyte:
Hows this? You can tweak this for desired response below 1Khz. Your desire to use one stage is a little unreasonable since you are already asking too much. Here in America we call that putting 50 lbs of manure in a 10 lb bag. Use a quad opamp and 0402s or tell your boss/client about the manure conundrum and ask him how he would solve it @grouchobyte |
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