Currently I'm working on a project to record ELF (Extremely Low Frequency) signals from a receiver. The highest frequency of interest is about 30 Hz and the system should have a flat frequency response. I'm looking for a simple solution using an 8 bit ADC and I wonder how the resolution can be improved using oversampling and to alleviate the requirements for the analog low pass filter before the ADC. Without oversampling and a sampling rate of 64 sps a rather steep analog low pass filter is needed to avoid aliasing. With oversampling and a gain of 3 more bits the sampling rate is 2^(2*3) times 64 sps = 4096 sps and the analog filter before the ADC is much easier to realize. The problem is to convert the 4096 sps signal back to 64 sps using a digital low pass filter and downsampling.
From what I read so far, the attenuation needs to be almost 68 dB at 32 Hz for 11 bits but at the same time signals up to about 30 Hz should pass. It seems a very steep digital low pass filter is needed. I tried to do some calculations with filter design software, but the required order is too high to get results. Is this really so hard to realize or am I missing something? Any help would be greatly appreciated.
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