I asked exact the same here one year ago: One year ago question I asked this question again here, because after one year, I have learned more about DSP and am thinking ways to improve it.
Filter description
Filter input data rate is 64kHz
Filter output data rate is 0.5Hz - 2Hz
The filter is preferably to have bandwidth of 1mHz with very sharp transition area, since only DC is needed and the noises at 0-5Hz is large.
The answer I got
The averaging data is the best DC I can get. I bought this answer. However, it now becomes plausible to me, since a filter always have a bandwidth and frequency responses. Averaging is essentially a FIR with no superior characteristics.
Questions
Shall the filter be designed in this way: 9 stages of halfband filters(64kHz -> 125Hz). Followed by three stages of CIC filters with R=5. Then the output will be fed into a 1st order IIR filter?
Maybe 16 stages of halfband filters? (64kHz -> 0.9765625Hz) And then be fed into an IIR filter.
How shall the filter be designed?