Inverting Sallen-Key low pass filter

S-K is inherently non-inverting. Maybe you mean the MFB (multiple feedback) topology.

TI's FilterPro program is handy. It accounts for opamp GBW, and can select standard R and C values.

Google MFB filter

It appears the MFB filters are bandpass though, not the low pass I was looking for.

Is there any configuration of two pole active low pass filter that is inverting?

Thanks,

Robert Stevens

MFB can be Low Pass. For example see fig 2. page 7 of this:

piglet

Thanks. That is exactly what I was looking for.

Robert Stevens

With a 5 volt power supply, you can make a fairly decent MFB filter using the TL431 in a pinch!

The state variable filter can be made both inverting and non-inverting. but it takes three opamps.

George H.

MFB filters can have pretty high component sensitivity if you do them wrong. Unity-gain SKs with equal or nearly-equal resistors are very well behaved unless the Q is large.

It's pretty educational to re-calculate the filter with components at the tolerance limits, of course including the tempcos of the Rs and Cs, and the voltage coeffients of the Cs.

Cheers

Phil Hobbs

High-Q active filters will always be twichey, some less than others.

Apparently there's some magical interaction inside an LC filter that tames the sensitivities.

And get a lifelong exclusion from polite society.

The magical interaction is the square-root dependence, i.e. a sensitivity coefficient of 1/2. You never get below 1.0 for active filters AFAIK.

omega_0 = 1/sqrt(LC)

Q = sqrt(L/(R**2 C))

Cheers

Phil Hobbs

One of my daughters used to live in the Vieux Carr'{e}, at Dauphine & St. Peter. She said that it was very public-spirited of some of the male tourists to wear T-shirts covered with various misogynistic slogans, because it saved women the unpleasantness of actually meeting them.

Cheers

Phil Hobbs

As you suggest, the S-K component sensitivities for Q scale with Q, i.e. they get fairly bad for Qs higher than a small fraction. This is consistent with Budak (see p. 181).

I disagree about MFB sensitivities - wo and Q sensitivities for the MFB are |S|

New Orleans attracts all the redneck party-types from Texas to Florida.

It's whatever you want, in two poles. Take John's advice and get FilterPro from TI. It'll do whatever SK or MFB filter you want.

Yes. Look!

was

is

With regard to "matched" doubly terminated passive filters, they are also i nsensitive because this is the point of "maximum transfer of power." The sl ope of deltaP is zero, or near zero, for most of the passband. Therefore, any deltaL or deltaC with respect to the zero, or near zero, slope of power transfer (deltaP) is necessarily insensitive.

Of course, it is all relative. The nearer the natural modes are to the jw axis, the more sensitive it will be, ceteris paribus. "Unmatched" LC filte rs are more sensitive than matched ones, again ceteris paribus.

By matched I mean there is ideally no reflected power in the passband aside from the unavoidable dips in the eps ripples.

I've never gone through the math of MFB filters, but I don't think we actually disagree. I was comparing MFB filters "if you do them wrong" with "unity-gain SKs with equal or nearly-equal resistors".

The OP seemed to be building his first active filter, or very nearly, so a few cautions seemed in order.

Cheers

Phil Hobbs

"Passive and Active Network Analysis and Synthesis" by Aram Budak, Houghton-Mifflin 1974. It was old when I was told off to buy it for a class, but I still pull it out from time to time today to get valuable information. If you can get it -- get it.

It goes into detail on the filter you're considering making.

(That was a great class. Usually gut classes make me want to barf, but that was an easy A that taught me something new every day I was in the class -- which was a godsend because I was taking the World's Hardest Class at the same time, and I needed something easy to offset it.)

OK I'll bite, what was your "Worlds Hardest Class"? (your caps.) (I got a C in organic chem.) George H.

OK I'll bite, what was your "Worlds Hardest Class"? (your caps.) (I got a C in organic chem.) George H.

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There's a reason the only "Honk if you passed xxx" bumper stickers the American Chemical Society sells is for pchem, physical chemistry :-) :-). I worked as hard or harder in pchem than any other class.

----- Regards, Carl Ijames