ADS1220: fdiff & fcm

The answers to your question are below in red.

1. Why are fcm and fdiff different?  Making the common mode filter 10x larger than the differential filter minimizes common mode to differential conversion.  See attached PDF.

If the desired cut-off frequency is the same, can I use fcm=fdiff?  The cut-off for the common mode should be set above the maximum frequency you want to measure.  Remember, at the cut-off the attenuation is -3dB, so you need to decide what maximum attenuation your input signal will see and set the filter cutoff accordingly.  The common mode filter cutoff needs to be set above the differential mode filter by setting the capacitor to minimize common mode to differential conversion.

2. Why does ti recommend Cdiff>=10Ccm? Can't Cdiff be less than Ccm?  See attached PDF.

3. Is there a recommended range of use of Cdiif and Ccm? Can I use ~ 1000uF? 1000uF is unusually large and probably a bad idea.  The ADS1220 has built in digital filters with very low cutoff frequencies.  The external filter is primarily meant to act as an antialiasing filter.  Look through the antialiasing section in the data sheet.  A typical filter would be Ccm = 100nF, Cdif = 10nF, R = 1kohm.

Art

common mode and differential filter.pdf

Thank you for your reply.