The accuracy of the internal reference of the XADC Initial Engineering Sample (ES) and General ES parts for Kintex-7 and Virtex-7 FPGA is 1.5% instead of 1% as stated in the device errata.
What effect does this have on measurements?
The 1.5% internal reference accuracy results in an additional gain error. For example, it could result in an additional +/- 2 degrees Centigrade error in Temperature measurement from the error specified in the device data sheet.
For the voltage sensors, the internal reference errata item could result in an additional 0.5% of error. If measuring 1V (e.g., Vccint), it could cause 5 mV extra of error from the error specified in the device data sheet. If measuring 1.8V (e.g., Vccaux), it could cause an extra 9 mV of error from the error specified in the device data sheet.
If this accuracy is not sufficient, the external reference can be used for better accuracy; see the XADC Pinout Requirements figure in the 7 Series FPGAs XADC Dual 12 Bit MSPS Analog to Digital converter User Guide (UG480) for details on using the external reference.
NOTE: The Internal reference is not supported for ES Lab parts. The internal reference of the XADC requires a trim during the production phase, but due to an issue this trim was not performed. This impacts the accuracy of the internal reference, which in turn affects the accuracy of all the XADC measurements. Therefore, the external reference must be used.