08-11-2015, 04:13
(07-11-2015, 12:37)AllyCat Wrote: Of course the optimum calibration range will depend on your location (very different peak light levels between UK and Australia) and I don't know how accurate/stable is the "1.1 volt" reference in the Arduino.
According to the ATMEGA328 datasheet the accuracy of the reference voltage is 1.1 ±0.1 V, which seems great (around 10%), but some sources refer that this accuracy is from chip to chip, and that this voltage (within the 10% margin) is stable for each chip, and almost nothing dependent of the temperature.
The measured reference voltage of the tested Arduino Nano was 1.089 V at an ambient temperature of 21 °C and the package temperature was 29 °C. When the package of the ATMEGA328 is cooled down with freeze spray to -18 °C the reference voltage measures 1.084 V. The temperature drift is therefore 100 ppm/°C.