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Old 13th April 2005, 04:44 PM   #1
Luke123 is offline Luke123  Canada
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Default Filter expert needed...

I need a filter expert on this.
This is a fourth order 10 KHz lowpass filter made of a pair of 2nd-order sections.
There is an upward departure from target at -80dB level.
Is this due to excessed phase rotations in the opamps?
Can this be cured with frequency compensation?
If so, how do you apply this with S/K and MFB type of lowpass filters?
If you can also help me with a link, it will be appreciated!

Luke
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Old 13th April 2005, 04:54 PM   #2
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What filter topology are you using? My Sallen-Key filters work fine.
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Old 13th April 2005, 05:13 PM   #3
Luke123 is offline Luke123  Canada
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Sallen-key or multi-loop feedback design...they have the same problem . Same problem with OP275, OPa213x, and others. All departs from perfect LP4 transfer functions.

Is your a dual 2nd-order cascade?
Tuned somewhere between 1 and 10KHz?
Did you measure it down to -100 dB?
I read something long time ago about this...don't remember where.

Luk123
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Old 13th April 2005, 05:31 PM   #4
Mr Evil is offline Mr Evil  United Kingdom
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It's due to finite op-amp bandwidth. Use an op-amp with a high GBP to minimize the problem.
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Old 13th April 2005, 09:44 PM   #5
Bricolo is offline Bricolo  France
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There has juwt been a nice article on the bob pease show about that
available on www.national.com

it's due to the opamp bandwidth, but optimal component values can give better results
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Old 13th April 2005, 10:29 PM   #6
Luke123 is offline Luke123  Canada
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Hey Broccolo!

Do you remember the title of the article?
Any cue available?

Luke123
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Old 13th April 2005, 10:41 PM   #7
Bricolo is offline Bricolo  France
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on national's website, do a searh on "bob pease show"
I think you have to register an account to have access to it

and it's not an article per say, it's a show video+sound, you need real player or windows media player
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Old 14th April 2005, 12:23 AM   #8
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What chips are you using - as has been said, ones with higher GBW are the go!

OPA627 is best! But you can use lesser chips if the alignment is low Q i.e. Butterworth or Bessel, Chebyshev give steeper cutoff but require more GBW and give a distortion spike just below cutoff frequency.

If it's for a quality audio use MFP will give lower distortion and best to use more sections of low Q depending on the application.

Hope this helps.
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Old 14th April 2005, 05:38 AM   #9
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Some more...

A rule of thumb is for MFP sections GBW should be > 100xGAINxfn (natural for section)

For Sallen-Key GBW should be > 100xGAINxQxQxQxfn so if Q is high it's very high.
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Old 14th April 2005, 01:27 PM   #10
Luke123 is offline Luke123  Canada
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Thanks for collaboration

When I started this thread, I just wanted to expose my problem in a very simple way. Bricolo pointed me out at the Bob Pease show link. (Thanks by the way.) But I have already seen it and this is what inspired me to write this thread actually. The picture is a screen capture from a Webench simulation. I did this because I don't have the original measurements of the specific problem. Exposing the thread this way was much more simple for me.

Amplifier Guru : The two LPF2 in cascade had a F-Q values of 1300Hz/1.3 and 1500Hz/1.35. The tried Opamps were OP275 and OPA2134. I am aware of your law for needed bandwidth versus frequency/Q/gain and these opamps does not infringe this law.
Anyway I remember having tried it with AD745 with the same results.
I'm sure it have something to see with a custom frequency compensation for slowing down phase rotation in VHF.
But how?

Luke123
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