hi guys thanks for your reply. I will give you more details so you can help. My problem is i am trying to make a thermometer with a pt100 sensor.The sensor has an accuraccy of 0.15 degrees +/- and i wish to keep as close to that as possible. To make it work i need to use a constant current source to drop a voltage over it.Then measure it with a adc but because the resistance values at diffrent temperatures the resistance is given in ohms to 2 decimal places. if you dont measure it to 2 decimal places the accuracy will decrease alot. which means i need to be able to measure the voltage in milivolts to 2 decimal places to calculate the resistance to 2 decimal places. so my question about the refrence voltage is to get that accuracy with a adc i think need to use a adc with a 12 bit or higher adc to achieve that accuracy or use fairly low refrence voltage so i am wondering which is the best way to go if someone could help i would be greatful thanks