About the RFI input filters design

Question

Hi, 
 
 In "A Designer&rsquo;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&rsquo;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.

Arsol · Accepted Answer

Hi diverger 
 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)

BW DIFF (kHz)

BW CM (kHz)

AD8221

1

825

~600

1.855

39.59

BWCM factor

BW CM /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)

Arsol · Answer

Hi diverger 
 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,

	Gain	-3dB BW(kHz)	Signal BW(kHz)	BW_DIFF(kHz)	BW_CM(kHz)
AD8221	1	825	~600	1.855	39.59
BWCM factor				BW_CM/21
-3dB BW factor			0.75% x -3dB BW	0.22% x -3dB BW

	Gain	-3dB BW(kHz)	Signal BW(kHz)	BW_DIFF(kHz)	BW_CM(kHz)
AD8221	1	825	~600	1.855	39.59
BWCM factor				BW_CM/21
-3dB BW factor			0.75% x -3dB BW	0.22% x -3dB BW

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About the RFI input filters design