# Precision resistor on ADC voltage reference pins

When reading an RTD temperature using an ADC, the following topology is usually used:

Most guidelines insist that a precision resistor (0.1%) be used for RREF (inside red rectangle). I believe the reason for this practice, is to use the full range of the ADC device (correct me if I'm wrong). Would any problem arise if I don't need to use the full range of the ADC and I use a normal resistor with 5% accuracy instead of a precision one? Keep in mind that a full scale system calibration will be performed and I have also performed all calculations to make sure nothing goes outside the acceptable limits of the ADC.

• ADC range usually means the difference between highest and lowest voltages it reads. Are you using 'range' to indicate the precision with which you are expecting to read it, so the number of bits you can have confidence in? A calibration will take out initial error in the resistor, but a 5% resistor may have much worse resistance tempco than a precision one. – Neil_UK Mar 2 '16 at 13:41
• @Neil_UK Now that you mention it, I probably got something wrong about the 'range'. What is resistance 'tempco'? – hadez Mar 2 '16 at 15:27
• It's temperature coefficient of resistance. Typical 1% resistors may be 200ppm per degree, 5% may be worse. The best precision resistors may be quoted in the 50ppm to 15ppm range depending on cost. For comparison (why you don't use tracks for current shunt resistors) copper has a tempco of 0.4% or 4000ppm per degree! – Neil_UK Mar 2 '16 at 16:47

The designator of the component should be a clue-$R_{REF}$.. it's the REFERENCE resistor to which your RTD value is compared.