When the output is less than 2.61V it merely means the current is flowing in the opposite direction.

If using a 10 bit ADC and 0-5V, each bit represents 1/1024 of the total so 2.61V should give you 534 bits (0.004883V per bit). For readings > 534, subtract 534 and multiply by 0.004883 to get the voltage then divide by the chips sensitivity (mV per Amp). For readings < 534, subtract the reading from 534 and and multiply by 0.004883 to get the voltage then divide by the chips sensitivity (mV per Amp).