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You can find how to build the decimation filter in many DSP books. Typical implementation involves ROM/RAM based controller plus a MAC unit. The filter coefficients can be easily found in MATLAB filter design tools.
A decimation filter uses a Cascaded-Integrator-Comb section
followed by a FIR section. The CIC section decimates down to 4
times the output sampling frequency and has a response of the
form (sin x over x)^n, n being higher than the order of the analog
section. The FIR can be any linear phase design, and is used for
antialias pourposes. If the clock frequency is less than, say 10 MHz,
power is very low (uW).