my ADC was 1 bit modulator
so I am confused for multibits data (after decimation)
Do you understand decimation filter ?
Most simple decimation filter is cic filter. Frequency characteristic of cic filter is same as moving averaging filter.
https://en.wikipedia.org/wiki/Cascaded_integrator-comb_filter
Output data of your ADC are +1 or -1, since resolution of your ADC is 1bit.
Consider output code 1,1,-1, 1,-1,-1, 1,1,1, -1,-1,-1,....
And assume moving average of 3 points.
Resulting data after cic decimation filter will be, 1/3, -1/3, 1, -1, ...
Data reolution is now 4 values, -1, -1/3, 1/3, 1.
If more points of averging are used, data resolution will be larger.