Hi,
Talking about the onchip digital decimation filter, page 20 of the datasheet says "This ΔΣ converter feature drastically reduces the complexity of the analog antialiasing filters typically required with nyquist ADCs."
I assume the antialising filter of reduced complexity is the first order RC filter placed before the each ADS1299 input. I was under the impression that this RC filter was mostly used for RFI attenuation, as from memory, the values of R and C in the Eval Board give a cutoff frequency in the couple of kHz. I never assumed it was used specifically for antialising as a cutoff frequency this high would not be very useful for applications with a sampling frequency under 4ksps.
For instance, a simple test with a sinus wave at 180hz sampled at 250 sps gives you aliasing at 70hz as expected (results in figure below). (recording of 180hz sinus wave with ADS1299, gain of 24, 250sps)
This is why I am a bit confused by the reasoning of a simple passive RC filter in the front-end providing RFI protection and AA filtering. Futhermore, using larger values of C for lower cutoff frequency would degrade the CMRR, as lower reactive capacitance worsens the CMRR.
I am also a bit confused as to how the digital decimation filter with zeroes at multiples of the datarate will actually help with AA filtering.
Best regards,
Mikaël