Tl431 based filter

Nope. It's a convenient one-input (2.5V above ground) OpAmp. I used it a lot in the 1977-87 time frame when I was at GenRad, in opto-isolated feedback loops. Worked great! ...Jim Thompson

Interesting, I wonder how that would work out as a notch filter?

I use a TL431 in a cascode configuration and set the input from a remote part of the circuit, sort of like a modulator circuit. It works very good and is compact but this circuit operates in a line power noisy environment and it would be nice if I could fit it to notch the 60hz.

We'll have to investigate further to see if that would actually work.

Nice idea btw.. I've always like doing a lot with minimum part count.

Jamie

The only possible "gotcha" I can see, if it could be called that, is that the "Ref" pin draws significantly higher current than the input of an ordinary bipolar opamp; uA vs. nA. The filter design software I use often chooses very large values for the feedback resistor going to the inverting input in the MFB configuration to keep the capacitances small, so a compromise will have to be made here.

So use your software, then realize, in ANY active filter: R*C=constant, so scale trivially ;-) ...Jim Thompson