AR# 51846

7 Series XADC - Does the selection of Internal or External Reference impact accuracy?


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. 

The Gain Error is +/-0.5%, and if using the on-chipreference it can vary +/-12.5mV (within -40degC to 100degC range) which is avariance of +/-1%, so the total gain error would be 1.5%. This applies to the Analog input.

Note: to ensure 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.

AR# 51846
Date 03/16/2017
Status Active
Type General Article