your sensor gives Vcc/2 when I=0
and for +20A DC 100x20=> 2000 +Vcc/2
for -20A DC -(100x20)=> - 2000 +Vcc/2
if Vcc=+5V => output =2,5V +-2,0V at +-20Amps
With ADC 10 bits => delta max= 40 Amps for delta 4V
=> 1023*4/5= 819 points for 40 Amp peak to peak => 20 points / amp or resolution 1/20 Amps => +-50mA
We suppose , AC is sinusoidal ! (no distortion with hamonics)
if signal is clean,
you can take measures at 0.1 mS (or less) means 200 measure for one periode 50Hz,
to catch the maxima and minima value => peak to peak value
RMS is proportional .. and adjust your calibration coefficient to read Amps or xx.x amps
i did a similar measure but for voltage 230V AC.. use of a transfo 230V/9V and scale the signal around +2,5V DC.
.