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Old 1st May 2012, 02:49 AM   #1
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Default Calculating LPF Fc for Generic Filter

I come across this basic design in most DAC datasheets but they never discuss the calculations necessary to deviate from their suggested values so that the Fc can be adjusted for other applications. I've seen a few app notes on filter design but never this exact design- there's always a little difference. FilterPro does not have this design either. Does anyone know where to find the formulae needed to make such calculation for this exact design?

Thanks,
Stu

lpf.jpg
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Old 1st May 2012, 03:49 AM   #2
TheGimp is offline TheGimp  United States
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I think that is a Bessel, not Butterworth.

You may be able to find standard equations for it if you search for Bessel.

I prefer to do a sim in LTSpice. It is quick, accurate, and easy to modify and retest.
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Old 6th May 2012, 02:11 PM   #3
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Quote:
Originally Posted by TheGimp View Post
I think that is a Bessel, not Butterworth.

You may be able to find standard equations for it if you search for Bessel.

I prefer to do a sim in LTSpice. It is quick, accurate, and easy to modify and retest.
Given the Q of the circuit, this is a Butterworth filter since the Q of Bessels do not go that high.

To answer my own question, using the Cirrus App note AN48 on filter design I was able to replicate the values in the filter attached earlier after multipling C1 (the 3300 pF cap in my initial post) by 2. This is needed because having the cap between the differential outputs is equivalent to having two caps in series together which is essentially how it is configured. So for whatever value you use C1, you would essentially be using two caps twice its size when having one on the positive output to ground and one on the negative output to ground as shown in the attached schematic.

Tried LTSpice but couldn't figure out what settings to use for signal, net names, etc. Thanks anyway.

Stuart
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