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Design >> Analog Design >> filter stability
https://designers-guide.org/forum/YaBB.pl?num=1205389699

Message started by aaron_do on Mar 12^th, 2008, 11:28pm

Title: filter stability
Post by aaron_do on Mar 12^th, 2008, 11:28pm

Hi all,

i'm designing an active RC filter. I have not yet added compensation to the opamp, but I was wondering if there is any need to. The op amp is after all used in a filter where the caps and R's determine the phase shift and gain in the closed loop. So anyway my second question is, is there a way to do a closed-loop stability check?

thanks,
aaron

Title: Re: filter stability
Post by Frank Wiedmann on Mar 13^th, 2008, 3:37am

You need to make sure that the loop in your specific circuit is stable. The question if your opamp would be stable in e.g. a unity-gain configuration is not important if you only want to use it for this specific circuit.

You can use Spectre's stb analysis for checking loop stability. This has been discussed a number of times in this forum, e.g. at http://www.designers-guide.org/Forum/YaBB.pl?num=1201763532 and http://www.designers-guide.org/Forum/YaBB.pl?num=1204029625.

Title: Re: filter stability
Post by thechopper on Mar 13^th, 2008, 6:20pm

Hi Aaron,

I would think you still need to add compensation to the opamp. The reason is that the frequencies at which your RC filter will be will behave as such should be lower than the ones at which the opamp introduces significant phase shifts (opamp poles). In other words: the frequencies associated with the RC closed loop phase shifts should be such that the opamp open loop gain is large enough. Therefore opamp open loop gain singularities should be at higher frequencies than the RC time constant.
Loop stability should be guaranteed by setting the dominant pole in the opamp open loop gain, which cannot be guaranteed if the requirement mentioned above is fulfilled.

Hope this helps
Tosei.