http://www.musashinodenpa.com/arduino/ref/index.php?f=0&pos=2244
void loop() {
// read the input on A0 at default resolution (10 bits)
// and send it out the serial connection
analogReadResolution(10);
Serial.print("ADC 10-bit (default) : ");
Serial.print(analogRead(A0));// change the resolution to 12 bits and read A0
analogReadResolution(12);
Serial.print(", 12-bit : ");
Serial.print(analogRead(A0));
デフォルトのAD変換が10bitなので、
analogReadResolution(12)を入れて、12bitにする。(Arduino dueの場合)