If you will use 10-bit ADC to read the voltage on 250R resistor, why would you need an opamp?250R will give you 5V at 20mA max.and, 1V at 4mA lowest.Your ADC reading will be from 205 to 1023.And remember, most 4-10ma sensors provide lower than 4mA and higher then 20mA; Mostly, 2mA to 25mA is the field.So you may want to keep your voltage range between 0.5V and 4.5V. I would not go for 5V at max mA. You should make it 4.5V at 20mA. So if it goes higher than 20mA you still have room to detect it.