Help Reqd in step response of a LOW PASS filter..

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anishsingh

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Hi , I have tried to make a low pass filter of sallen key topology with a folded cascode OTA + a common drain stage and resistive feedback. The frequency response is coming out to be correct and phase margin of around 70 deg. But giving a step i/p from 0 to 1 volts, i get a spiky response which settles to its initial value, which seems to be wrong !

I wanna know wat's wrong there ! A good phase margin should mean a little overdamped rise to the higher step value and NOT a settling down to lower level !!

Plzz Help !!
 

If I understand right, you are talking about a completely erratical time domain response, not a minor deviation? Can be hardly discussed without seeing circuit details.

In the meantime, you may want to inspect the waveforms at individual circuit nodes. Hopefully, this should be sufficient to understand the problem's nature yourself.
 

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