Sean Schouten wrote...

>I have attached some
>screenshots so you can see.
>
>3k.gif = perfect sine @ 3Khz, http://seantech.offside.fromadia.com/3k.gif
>30k.gif = perfect sine @ 30Khz http://seantech.offside.fromadia.com/30k.gif
>300K.gif = more like a triangle @ 300Khz.
>http://seantech.offside.fromadia.com/300k.gif
>
>filter_and_response.gif = my current *test* design, and it's simulated
>responce characteristic.
>http://seantech.offside.fromadia.com/filter_and_response.gif
>
>Why on earth does my output signal deform at 300Khz? I am overlooking
>something.

Aha!!! What you're seeing has absolutely NOTHING to do with the
filter phase shift: what's happening is the opamp model you're
using is undergoing slew-rate limiting at that high frequency
(just like a real opamp would); it can't swing its output fast
enough.  That's why the sine wave is degrading into a triangle
wave.

I've been doing the "analog circuits for food" thing for almost a
third of a century; trust me, phase shift is not a factor here.

The cure, of course, is to use a faster opamp if you're going to
be generating frequencies that high.

Dave D.

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