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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]

## Top Replies

• 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…
• 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…

• 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

Regards

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.

• 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,