Low pass filter design using filter wizard

Hello,

I was recently trying to design a LPF using the filter wizard. My passband frequency is 20Hz (-3dB attenuation) and my stopband frequency is 80kHz (-80dB attenuation). I keep getting "No opamp found that meets the requirement". I want a design optimized for low noise and the only time I am able to get that is when I increase the passband frequency to about 110Hz. Is there some fundamental issue when the difference between the passband and stopband frequencies are so high? What is the cause for the design tool saying "No opamp found that meets the requirement"?



Grammatical error
[edited by: mohan at 8:08 AM (GMT 0) on 10 Aug 2020]
  • +1
    •  Analog Employees 
    on Aug 14, 2020 2:50 PM 3 months ago

    Sorry you are running into this.   Here is what is happening.

    Background:

    We use the Sallen Key filter circuit topology as our default.  This topology has several good properties, but a major drawback of this topology is that attenuation degrades at high frequencies.  This is because a small amount of the input signal goes around the op amp through the passive network rather than through it.  So for example if desired attenuation is -80dB, only 1/10000th of the signal needs to go around the op amp for it not to reach this design goal.

    This issue is the worst for filters with only one stage.  For example in a 4th order filter, the attenuation gets spread across two stages, so for -80dB attenuation, each stage only has to provide -40dB attenuation.  1st order filters also don't have this issue, so 3rd order filters (i.e. 2nd order stage + 1st order stage) also don't have this issue.

    Why you see what you are seeing: 

    The filter wizard checks for the upturn at higher frequencies and rules out amplifiers that can't meet the attenuation requirements.  I think it may check to 100x the stopband frequency.  So in your case, the tool is trying to find an op amp that can provide -80dB attenuation at 8 MHz.  This requires an op amp with 100's of MHz of GBW, maybe even a GHz, and we run out of options in our portfolio.  Unfortunately once the tool can't find a solution, it errors out, and you can't do anything in the components screen.

    How we should improve our tool:  

    There is another topology that is available in the Filter Wizard: the multiple feedback, which does not have this issue.  So when needing to design a filter with really deep attenuation from one stage, that's the better topology.  The filter tool should be automatically switching over to the Multiple Feedback topology in this situation, or at least give you the option to do so when you're getting the error.  There are plenty of op amps that can handle your design requirements in the Multiple Feedback configuration.

    Workaround for you in the meantime:  

    Step 1:  First avoid the error in the components screen.  You can see in the specifications screen that your design requirements result in a 2nd order filter.  You need to get the tool to generate this, without checking for -80 dB attenuation at such high frequencies.  You can do this by lowering the stopband requirement to 60 kHz.  This will allow you to go to the components screen without error.  

    Step 2: Then change to multiple feedback.   Once you are in the components screen, the tool will automatically pick a very fast op amp in the Sallen Key configuration.  But you can use many other op amps if you use the Multiple Feedback configuration instead.  To fix this, choose "I want to choose".  Then change the Implementation from "Sallen Key" to "Multiple Feedback".  Then choose your op amp - you should have lots of options now.    

    Apologies again that you run into this and have to hack the tool to get it to give you a good answer.

  • Hello,

    Thanks a lot for the detailed reply :) I am so sorry I took so long to see this message, the notification got lost in my email. Thank you for your valuable inputs. Will try the same.