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Old 5th July 2011, 02:29 PM   #1
graygem is offline graygem  United States
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Default multiple feedback low pass filter question

All the multiple feedback lowpass filters I see are inverting type. Is it possible to do a non-inverting multiple feedback low pass filter?
Just something that I keep asking myself.
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Old 5th July 2011, 03:13 PM   #2
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No, MFB filters are inherently inverting. If you're thinking of trying to input signal to non-inverting input of MFB filter, it won't work correctly. You must correct signal inversion elsewhere in the design. One way to accomplish that is to use another inverting circuit to "un-invert" final output, could be an inverting buffer or amp, or in the case of a 4th order lowpass for example, use two MFB filters in series, I've done that in some of my projects.

Mike
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Old 5th July 2011, 03:35 PM   #3
graygem is offline graygem  United States
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Thanks, that was my thoughts as well.
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Old 5th July 2011, 04:31 PM   #4
graygem is offline graygem  United States
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So, how would I make an inverting unity gain amp with a 28Hz roll off in front? Below is the non inverting buffer they use. What should I do to make into inverting amp?
unity.jpg
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Old 5th July 2011, 04:40 PM   #5
DF96 is offline DF96  England
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I may have misunderstood the question, because the answer is obvious? Use an inverting op-amp configuration, with a capacitor in series with the input resistor.
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Old 5th July 2011, 04:52 PM   #6
graygem is offline graygem  United States
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I need an inverting amp from the non inverting pictured. I know it is very simple but I'm still learning.
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Old 5th July 2011, 05:09 PM   #7
DF96 is offline DF96  England
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OK. Like I said, use the inverting configuration with a capacitor.
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Old 5th July 2011, 05:22 PM   #8
graygem is offline graygem  United States
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Like this?
unity2.jpg
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Old 5th July 2011, 05:26 PM   #9
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Here ya go.

INVERTING OPAMP.JPG

Mike
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Old 5th July 2011, 05:34 PM   #10
Davey is offline Davey  United States
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I thought you were interested in a non-inverting.....multiple feedback.....and low-pass filter?

Now, you've got an inverting.....non-multiple-feedback....high-pass filter.

Cheers,

Dave.
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