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ADC input filter

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OM222O:
Hello everyone.
I'm using a 24 bit ADC for a project.
I was going through the datasheet again when I saw this note:


I was curious so I had a look at another data sheet from a similar ADC (although 16 bit) and found a very similar note!


Now I'm not sure why Cdiff should be at least 10x higher than common-mode caps? I have previously used the ADC with 2 1k resistors and 3 100nF capacitors without any issues:

I will probably use 1k resistor array again, and bump up the Cdiff to 1uF (the application is purely DC, so cutoff frequency doesn't really matter) or maybe reduce the resistors to 100ohm for lower error. Any comments on why they insist on the value of capacitors being an order of magnitude different? Thanks!

T3sl4co1l:
It's simply a caution as stated; if you don't mind a little common mode error at high frequencies, it's alright.

Most applications for these things are reading nearly DC anyway, so even the 10Hz internal filter that many offer, can stand to be further supplemented with additional [digital] filtering.

Tim

David Hess:
The mismatch between the two common mode filters on the differential signal converts common mode noise into differential noise so the differential capacitance is made larger to reduce this effect.  If you are making measurements in the presence of large amounts of common mode noise, then a different circuit is called for to prevent this problem.

wraper:

--- Quote ---I have previously used the ADC with 2 1k resistors and 3 100nF capacitors without any issues:
--- End quote ---
That means signal was quite clean to begin with. Not only C8, C9 should have much lower capacitance, they should be closely matched. And the smaller the difference there is with C7, the closer they should be matched. Using X7R, X5R and similar unstable capacitors for C8, C9 is unacceptable unless it's X2Y cap (2 caps with 1 common terminal). R1 and R2 should be closely matched as well.

OM222O:
I have not heard of X2Y capacitors before! they seem to be great for this application. also why is 100nF too large? what value would you recommend?

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