In 7 Series XADC either an external or internal reference can be used.
What is the benefit of using the external reference?
The 7 Series Data Sheets contain accuracy specifications, but does not differentiate between which type of reference is used (the previous family data sheet had a specification with internal reference and another specification with the external reference).
The specifications in the data sheet are valid for an ideal reference of 1.25V, with both external or internal reference assuming the calibration is enabled.
The actual tolerance of the reference (internal or external) should be added to calculate the total accuracy.
Using the external reference usually delivers the most accurate result as the internal reference has a wider tolerance than the recommended external Reference ICs.
Reference ICs have a typical error ~0.2% and 50ppm/C.
The reference error impact to the temperature sensor is 1:1 in Kelvin. Therefore if the temperature is 100degC / 373K, a 1% reference error would be 3.73degC.
The Total Error*** for Temperature = Temperature Sensor Error + Reference Error @ -40C to 100C = +/- 4 degC + +/- 3.73degC = +/-7.73degC
The Total Error*** for an Internal sensor = Supply Sensor Error + Reference Error, for @ -40C to 100C => 1% + 1% = 2% Error. For Vccint = 1V, 2% Total Error = 20mV.
*** Total Error is calculated as a straight addition of error sources assuming a non Gaussian distribution giving the worse case.
For external Analog inputs the accuracy of the external reference will have a direct impact on the Gain Error of the XADC. In the datasheet we specify a maximum gain error of +/-0.5%.
If using the on-chip reference it can vary +/-12.5mV (within -40degC to 100degC range) which is a variance of +/-1%, so the total gain error would be 1.5%.
Note: to ensure that you are getting the most accurate readings, enable calibration and averaging. Also ensure the correct placement of the ferrite bead digital and analog ground.
See XAPP554 XADC Layout Guidelines for further information.