Electronics > Projects, Designs, and Technical Stuff
2nd Order low pass filter - Why a topology?
BartK:
--- Quote from: imo on May 07, 2019, 10:22:45 am ---The output does not go to 0V in single rail Vcc.
--- End quote ---
Ah well, I was aware of this. This is sufficient for the application, the signals that will mainly be used are 1-10V.
I had already devised a workaround for this, namely to raise the input voltage with a volt. Then after the opamp I wanted to remove this by reading with ADC. However, this results in a lower resolution with the ADC.
To keep the complexity and price of the circuit low, this has not been done.
Thank you for your good advice!
emece67:
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Benta:
It's quite simple: two first order filters in series do not equal a second order filter. Do the maths.
emece67:
.
T3sl4co1l:
The maximum Q factor for an RC network is 0.5 -- this limits how sharp the filter transitions from pass to cutoff. A higher Q is possible with passive LC or active RLC filters, giving sharper cutoffs (maximally flat (Butterworth), peaked (Chebychev), etc.), or more precise profiles (Bessel is very gentle, but just not quite as soft as an RC response, and most significantly so for high-order filters).
MFB is generally preferred for lower sensitivity to component variations. There are some other topologies which are preferred for high-Q filters, but this is only important in high-order and narrow band-pass/stop filters.
Incidentally, you do get one order almost for free -- the fact that the named filter profiles have poles positioned evenly around an ellipse, means that odd-order filters always have one real pole (and the rest as complex conjugate pairs). RC filters make real poles (this is the abstract mathematical reason why LC or gain is required to make a sharp filter), so you can solve for the pairs with active stages and throw in a single RC to finish it up.
The topology for that case can be found here (with calculators!):
http://sim.okawa-denshi.jp/en/Fkeisan.htm
Simply, it's another RC put in front of the usual RCRC filter. The values affect each other, so a different formula applies.
Tim
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