I occasionally review various online active filter design tools. Is there some detail in some document that describes where those min. GBW numbers come from. Specifically, if you take a 1MHz gain of 1 Butterworth 2nd order (single stage) design, and navigate to an MFB design and then try to change the amplifier, starting from this screen, hit change,
I then get this where at the bottom I see two min GBW numbers - the lower one seems too low for this design, any idea where that is coming from?
If I go back and activate the adjust for GBW, I would expect these min numbers to change, but they don't. Maybe that 1.77Mhz is where it is assuming a GBW adjust in the RC values whereas the 7.08MHz is not? It does not seem any of the available op amps to change to are anywhere near this low on GBW. The slowest I see is 8MHz so still well above the required min 1.77MHz.
I think this post should give you most of the background:
Given that this question has come up a couple times now on EngineerZone, probably a lot more folks have wondered about it, so it sounds like it would be good for us to add some explanation in the Filter Wizard tool itself. I'll file a ticket for us to add this as a new feature.
At the recommended bandwidth, we expect the filter to work well. At the required bandwidth, we expect the filter performance to be degraded, but probably stills work for some applications (i.e. there may be applications where the power savings justifies the worse performance.)
The "adjust for GBW" setting adjusts the R & C values to compensate for the degraded bandwidth as best we can. However this comes with other tradeoffs, so we don't want to push it too far. See the help (question mark next to that control) for more info. We see it as a method to make a working filter a little more precise rather than a method to transform a bad filter into a marginal one.
Thanks, that post is useful - and yes it is a judgement call where older tools use different judgements.