I am trying to understand the choices going into the digital filter design in the TI ADS1119: They give the filter response on page 18 of the datasheet, here is e.g. the curve for the 20 sample/s case:

The parts I think I (mostly) understand:

• There are two "notch filters" at 50Hz & 60Hz to filter out line noise
• The filter appears to have 22 coefficients, which together with the 400Hz sampling rate (judging by the frequency window they plot) almost results in the 20samples/s effective sample rate at the end.

The parts I don't understand:

• Why 22 coefficients and not 20, if the goal is to neatly get to 20 samples/s at the end? (assuming you want every sample to be used once)
• Why is the response at >60Hz not more uniform/flat? If I use e.g. the tool from here to design a filter with two "strong" (target -90dB) stop bands at 48.8-50.8Hz and 58.4-60.4Hz (tweaking the numbers until the generated filter had the bands properly centered), and a "weaker" (target -30dB) one for 80-200Hz, I get a response like this: I can see that their response overall seems to be slightly lower, but I don't quite get why - wouldn't moving around the zeroes necessarily increase the response in some areas?

Obviously, nobody here knows what the designers were actually thinking, but I am wondering whether there are considerations that I am missing that would explain the final form of their filter.

• Perhaps they are also taking a swing at the line harmonics? Line noise is rarely just the fundamental, you also want to kill 100/120 and 150/180. Commented Jun 22 at 11:28
• @Hilmar good point, thanks! At one point I considered something like that, but I saw that for some reason they don't hit the 180Hz peak (so I naively dismissed the idea as wrong...). But I now realize that at least the 100/120Hz peaks are properly supressed. Given that odd harmonics are usually stronger, I would have expected the 180Hz peak to be more prioritized, but I guess they had other things to consider as well Commented Jun 22 at 11:52