16-bit ADC accuracy

Continuing the discussion from analogReadResolution 16 bit on certain pins? by @bitshift

When I set analogReadResolution to 16 (or anything higher than 12), and connect an analog pin (like A05 or A06) to the 3.3V pin with a 10k resistor, every 10 times or so I reset the Dash, I see readings around 3.27V instead of 3.30V. I see a similar ~1% drop when measuring external sources like a 3V battery. With 12-bit resolution (and below), the readings are more consistent across resets.

For example,

void setup() {

void loop() {
  Dash.snooze(500); // optional

should print around 65535 (=3.300V), but if I reset the Dash a few times it will print values around 64914 (=3.269V) until reset. I once saw the Dash report values around 3.25V until reset.

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