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About the RFI input filters design

Category: Datasheet/Specs

Hi,

In "A Designer’s Guide to Instrumentation Amplifiers" 3rd, p. 5-14, it recommends differential bandwidth of 10 times the signal frequency. And common-mode filter capacitors 10% the value of  C2 or less. So, the common-mode bandwidth should be at least about 20 times the differential bandwidth. But latter, it says:

The common-mode bandwidth should always be less than 10% of the in-amp’s bandwidth at unity gain.

That means the differential bandwidth should be less than 0.5% of the in-amp's bandwidth at unity gain, and the signal bandwidth only 0.05% of the unity gain bandwidth, right? Won't this be too small?

Thanks.



typo
[edited by: diverger at 6:14 AM (GMT -4) on 14 Mar 2024]

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    •  Analog Employees 
    •  Super User 
    +2 verified

    Hi  

    Thank you for reaching out and sharing your questions with the team.

    To be able to understand properly this inquiry, I would like to ask several items:

    • May I know if you are asking these to…
    •  Analog Employees 
    •  Super User 
    in reply to diverger +1 suggested

    Hi  

    Thank you for pointing this out; It's quite misleading on the guideline #2 of the cookbook approach, On page 5-14 of the Designer's guide and the only point here is when choosing an RFI filter…

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  • Hi  

    Thank you for reaching out and sharing your questions with the team.

    To be able to understand properly this inquiry, I would like to ask several items:

    • May I know if you are asking these to design an RFI circuit?
    • And what instrumentation amplifier will you use? i.e. AD8220, AD8221, etc..

    Regarding your question, I made tabular data to interpret better what is being recommended by the Designers Guide.

    Let us use AD8221, and here is the information I got from the Datasheet.

     

    Gain

     -3dB BW(kHz)

    Signal BW(kHz)

    BWDIFF(kHz)

    BWCM(kHz)

    AD8221

    1

    825

    ~600

    1.855

    39.59

    BWCM factor

     

     

     

    BWCM/21

     

     -3dB BW factor

     

     

    0.75% x -3dB BW

    0.22% x -3dB BW

     

    EC table shows the -3dB BW for Gain=1

    TPC Plot AD8221 Datasheet

    I hope this information will help you.

    Regards

  • Thanks for the answer.

    Yes, I'm considering the RFI design for INA.

    The values in the table are aligned with the docs except the 'signal BW'. The doc says:

    A differential bandwidth of 10 times the highest signal frequency is usually adequate.

    Then if I obey this rule, the signal BW should only be 185Hz for your 1.855KHz BWdiff, right?

    Thanks.

  • 0
    •  Analog Employees 
    •  Super User 
    in reply to diverger

    Hi  

    Thank you for pointing this out; It's quite misleading on the guideline #2 of the cookbook approach, On page 5-14 of the Designer's guide and the only point here is when choosing an RFI filter, your C2 must be 10 times larger than your C1.

    Regards,

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