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yvkrishna Senior Member Offline Posts: 117	any passive r-c circuit which can filter CM noise in band. Oct 29^th, 2014, 2:38am Hi, I have a Diff signal and CM noise in band, is there any passive circuit which can filter the CM noise component without filtering the DM signal component. Thanks, yvkrishna
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: any passive r-c circuit which can filter CM noise in band. Reply #1 - Nov 2^nd, 2014, 7:12pm Process the signal though a differential amplifier that should have significant CMRR characteristics. Keep the system fully differential end to end in any and all analog parts
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yvkrishna

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Posts: 117

Re: any passive r-c circuit which can filter CM noise in band.
Reply #2 - Nov 2^nd, 2014, 8:38pm

@loose-electron,

differential amplifier is probably the best way to address this, however my CM noise swing is about 4X larger the min. detectable DM signal present.

However I wonder if there exists any passive circuit which can do this job?
When I search for this on google I only get Commonmode choke.

I have also tried using a T network (shown in the pic)which offer different impedance for CM and DM picture, even here I am not able to achieve CM bandwidth < DM bandwidth.

looks like there is some fundamental limitation in what I am trying to achieve using passives.

Thanks,
yvkrishna

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: any passive r-c circuit which can filter CM noise in band. Reply #3 - Nov 3^rd, 2014, 4:59pm consider a common mode choke then. External to the chip if needed, or even internal to the chip if you have the freedom of doing some electromagnetics inside the chip. Will depend heavily on frequency range needed and the consequent size of the device.
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yvkrishna Senior Member Offline Posts: 117	Re: any passive r-c circuit which can filter CM noise in band. Reply #4 - Nov 3^rd, 2014, 10:02pm I doubt if there exists any simple r-c circuit which does this. Or is this impossible without using electromagnetics ?
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: any passive r-c circuit which can filter CM noise in band. Reply #5 - Nov 6^th, 2014, 4:01pm plenty of active circuit methods does it have to be passive? If so I think you are going to need some electromagnetics
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hiSkill_11 Community Member Offline Posts: 35	Re: any passive r-c circuit which can filter CM noise in band. Reply #6 - Sep 8^th, 2015, 11:57pm How about using diff amp with high DM BW and low CM BW? yvkrishna wrote on Oct 29^th, 2014, 2:38am: Hi, I have a Diff signal and CM noise in band, is there any passive circuit which can filter the CM noise component without filtering the DM signal component. Thanks, yvkrishna
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: any passive r-c circuit which can filter CM noise in band. Reply #7 - Sep 10^th, 2015, 10:22am Have you researched the concept of "Instrumentation Amplifiers" ??? Getting rid of big common mode signal content is what these are designed for. Research time.
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