Inverting Sallen-Key low pass filter

Can someone please describe the circuit topology for an inverting two pole Sallen-Key (or similar) low pass filter, or point me to a reliable diagram?

There are plenty of circuits online for the non-inverting type, but I am unclear about how the inverting type is built.

I would also like to know how to caluclate the component values for

500Hz cut-off, if this process differs from the non-inverting variety.

Robert Stevens

Reply to
Robert Stevens
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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

Reply to
John Larkin

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

Reply to
Robert Stevens

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

piglet

Reply to
piglet

Thanks. That is exactly what I was looking for.

Robert Stevens

Reply to
Robert Stevens

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

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Reply to
bitrex

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

George H.

Reply to
George Herold

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

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Dr Philip C D Hobbs 
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Reply to
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.

Reply to
John Larkin

And get a lifelong exclusion from polite society.

Reply to
John Larkin

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

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Reply to
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

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Dr Philip C D Hobbs 
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Reply to
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|

Reply to
Frank Miles

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

Reply to
John Larkin

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!

Reply to
krw

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.

Reply to
Simon S Aysdie

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

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Reply to
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.)

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Tim Wescott 
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Reply to
Tim Wescott

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

Reply to
George Herold

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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Carl Ijames

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