I have a question about the design of the anti-aliasing filter for the ADS1299 and in the ADS1299EEG-FE. The ADS1299 data sheet shows the on-chip decimation filter provides some anti-aliasing protection.
But for the minimum output sampling rate of 250Hz, frequencies components around Fmod = 1.024 MHz and its harmonics still exist and must be eliminated through additional external anti-aliasing filters. On the ADS1299EEG-FE, each channel has a single pole RC filter:
These filters have a cutoff frequency of 1/(2*pi*4.99k*4.7n) = 6.77 kHz With -20dB/decade attenuation from a single pole filter, this anti-aliasing filter only provides approximately -45 dB of attenuation around the 1.024 MHz range. Therefore, a 0.2mV 1.024 MHz frequency component would produce 1.13 Vpp (noise floor for 250 sampling rate and Vref =4.5) that would be aliased back into the pass band.