OPA2333: filter

ARJUN P.BHASKAR

Part Number: OPA2333

Dear sir,

If i have to design a fourth order LPF filter oof 350 KHz by cascading two LPF ,what should be the frequency of two LPF filter?

over 2 years ago

0 Michael Steffes over 2 years ago

Guru 48155 points

That will depend on your desired shape. A 4th order Butterworth will be different in the 2 stages than say a 4th order Bessel. The TI online filter tool can give you those target 2nd order stages.

0 Marek Lis over 2 years ago

TI__Guru 61340 points

You may launch the TI online filter design tool using following link:

https://www.ti.com/design-resources/design-tools-simulation/filter-designer.html

0 ARJUN P.BHASKAR over 2 years ago in reply to Michael Steffes

Prodigy 245 points

Thanks for the reply.

I have designed fourth order LPF filter theoretically by choosing two individual second order filter.The cutoff frequency i have chosen for both filters are 350khz but i have taken different quality factor and coefficients .i.e ,(1+a1s+b1s^2).

coefficient a1 =1.8478 a2=0784

b1=b2=1

q1 =0.54

q2=1.39

is this method correct?

0 Michael Steffes over 2 years ago in reply to ARJUN P.BHASKAR

Guru 48155 points

Well without recreating the legacy pole location algorithms, you might peruse this classic treatment in the context of the original BurrBrown FilerPro program,

http://www.ti.com/lit/an/sbfa001c/sbfa001c.pdf

The current TI online link Marek included is based off of this,

I myself use the old FilterPro program to find target poles then implement using more modern RC solution flows.

There is a mass of material behind that, but here is one article elucidating the min GBW product solution flow - what Filter are you trying to achieve?

https://www.planetanalog.com/use-true-gain-bandwidth-product-to-estimate-required-margin-in-active-filters-insight-13/

0 ARJUN P.BHASKAR over 2 years ago in reply to Michael Steffes

Prodigy 245 points

Thanks for sharing the user guide of filter design.

my problem statement is given as follows

Design a fourth order butter worth filter with cut off of 350KHz.

by using ti design tool i got design values.

But i want to know how those values is selected?

+1 Michael Steffes over 2 years ago in reply to ARJUN P.BHASKAR

Guru 48155 points

So Arjun,

You are embarking down a well worn path of classic polynomials - A lot of this I think was developed in the 1920's on and then applied over the years - the reference book most folks used from the 1980's on is here - Ch. 3 I think is what you want,

https://d1.amobbs.com/bbs_upload782111/files_32/ourdev_573166.pdf

I know when I was developing the Intersil online active filter tool you have to constrain your solutions to a finite set of well defined shapes and then maximum order - I limited that to 6th order in the tool I developed (that was from looking at the extreme RC accuracy required as you go to higher order asking for extreme Q's) At that point, the target 2nd and 1st order pole locations emerge from a set of look up tables - from info that is in works like the one above.

0 ARJUN P.BHASKAR over 2 years ago in reply to Michael Steffes

Prodigy 245 points

Thanks sir, will contact you for further doubts.

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OPA2333: filter