What is SAMC21 AtoD accuracy with external VRef?

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I've got a custom board using SAMC21 and am comparing results across boards.

On different copies of the same exact design, I see a variation in the AtoD readings as I go from board to board.

 

I'm using an external reference (2.048V reference zener), which appears to be right on the money.

 

On some boards, the SAMC21 measures within a few millivolts.

On other boards, I'm seeing ~15mV error: with 2.048 VRef, 12 bits, that's 15 LSB counts off as I see it.

 

Is this normal? I'm struggling to understand the accuracy specs in the datasheet...

 

PS: I'm using standard ASF3 code, which appears to me to load the factory calibration values during initialization.

pondzone