The equation to convert the ADC code to the ECG reading is:
ECG Reading (V) = (VREF×[18 bit ADC code]) / (2^17×Gain Setting)
Where VREF = 1V and Gain Setting is programmable to 20, 40, 80, or 160 V/V .
The 18-bit ADC code is two’s complement so the right polarity needs to be considered. The ADC is really a bipolar input so the full scale range is +/-1V or 2Vpp but the equation above is single sided.