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manodipan Community Member Offline Posts: 83	UGB requirement for Bandpass filter Jul 14^th, 2009, 6:42am Hi all, I have a doubt on the UGB requirement for a Bandpass filter ...let's say center frequency is around 3MHz and Qfactor=1.5....what should be the requiremnt of opamp UGB ???is 100MHz UGB is okay??Thanks a lot...........
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: UGB requirement for Bandpass filter Reply #1 - Jul 14^th, 2009, 8:34am hi, i think it is depends on the architecture..but roughly you can say its around 20MhZ, but 100 is too much..it consumes lot of power. thanks, rajasekhar.
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aaron_do

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Re: UGB requirement for Bandpass filter
Reply #2 - Jul 14^th, 2009, 8:49pm

Hi,

an ideal op-amp based filter only has a feedback path. However, a real filter will have a feedforward and a feedback path. The UGB of the loop gain will determine up the what frequency the BP filter will behave ideally. Beyond the UGB of the loop gain, the feedforward path will start to dominate, and the filter transfer function will no longer follow the ideal transfer function.

So the UGB requirement of the loop gain is determined also by your interferer profile and possibly noise requirements.

Also remember that even if the transfer function continues to roll-off after the UGB of the loop gain, the linearity will not be good at these frequencies.

cheers,
Aaron

BTW as Raja mentioned, if you overdesign the opamp, you will pay for it with higher power consumption.

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ray_wang

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Re: UGB requirement for Bandpass filter
Reply #3 - Jul 23^rd, 2009, 9:33pm

raja.cedt wrote on Jul 14^th, 2009, 8:34am:

hi,
� �i think it is �depends on the architecture..but roughly you can say its around 20MhZ, but 100 is too much..it consumes lot of power.
thanks,
rajasekhar.

i don't think 20Mhz would be enough unless u use some Q-tuning techniques. The peaking is significant with only 20Mhz UGB.

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: UGB requirement for Bandpass filter Reply #4 - Jul 23^rd, 2009, 10:24pm hi, i assumed that sallen key filter so roughly i gave that no..but i don't think peaking will come...could you please explain more Thanks, rajasekhar,.
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ray_wang

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Re: UGB requirement for Bandpass filter
Reply #5 - Jul 23^rd, 2009, 11:03pm

raja.cedt wrote on Jul 23^rd, 2009, 10:24pm:

hi,
� �i assumed that sallen key filter so roughly i gave that no..but i don't think peaking will come...could you please explain more

Thanks,
rajasekhar,.

Peaking is proportional to biquad Q and inverse proportional to opamp GBW. See a paper on filter written by several Japanese in JSSC, Nov. 2007.

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