Please consider this measurement technique:
As you may know, the ATXMEGA (in my particular case the ATXMEGA16A4U) ADC can use several references.
For the image above, I am using AVCC/1.6 (generated internally) and later an external Vref (in my case 1.24V +-0.25%).
Using AVCC/1.6 as a reference, I'm measuring Vref and using Vref as a reference I'm measuring AVCC/10 (generated internally).
As you can see from the math above, in theory the multiplication of both values should be an exact value.
As you can guess, in the field it is not.
I would therefore like to know what tolerances I should apply to each of these measurements knowing that:
- I already compensated for their respective offsets by measuring an external ground using the ADC
- Vref is a very stable (checked with oscilloscope) tension reference
- I'm using a 250KHz ADC speed together with averaging on 8192 samples
- AVCC is extremely stable and not noisy (tps7a4901)
- I'm already accounting for the ADC INL (+-3LSB)
- Calibration values are loaded when initializing the ADC
My current guesses are:
- DIV10 accuracy(nowhere to be found in the datasheets)
- AVCC/1.6 accuracy (the same)
I should also add that using AVCC/2 as a reference lead me to even worst results.