user10025: I created a quite simple Arduino sketch:
void setup() {
  Serial.begin(9600);
}

void loop() {
  int sensorValue = analogRead(A0);
  Serial.println(sensorValue);  

  float voltage = sensorValue * (5 / 1023.0); 
  Serial.println(voltage);

  delay(1000);
}

I use the following circuit:

My multimeter measures 3.31 V, between the pin 3V3 and GND.
I get a sensorValue of 730 and a calculated voltage of 3.57 V. This is 8% higher that 3.3V. What is the tolerance by the AD-converter?

user368: The absolute accuracy of the ADC in the ATmega328P when its clock is within its optimal range is given in the datasheet as 2LSB. This means that the actual value could be anywhere between 728 and 732.
Of course this depends on the accuracy of VCC, since it is being used as the ADC reference. This is why supply voltage measurements should be performed using the internal bandgap voltage instead.

10025: <p>I created a quite simple Arduino sketch:</p>

<pre><code>void setup() {
  Serial.begin(9600);
}

void loop() {
  int sensorValue = analogRead(A0);
  Serial.println(sensorValue);  

  float voltage = sensorValue * (5 / 1023.0); 
  Serial.println(voltage);

  delay(1000);
}
</code></pre>

<p>I use the following circuit:</p>

<p><img src="https://i.stack.imgur.com/NT6sS.png" alt="enter image description here"></p>

<p>My multimeter measures <strong>3.31 V</strong>, between the pin 3V3 and GND.</p>

<p>I get a <em>sensorValue</em> of <strong>730</strong> and a calculated <em>voltage</em> of <strong>3.57 V</strong>. This is 8% higher that 3.3V. What is the tolerance by the AD-converter?</p>
 368: <p>The absolute accuracy of the ADC in the ATmega328P when its clock is within its optimal range is given <a href="http://www.atmel.com/images/Atmel-8271-8-bit-AVR-Microcontroller-ATmega48A-48PA-88A-88PA-168A-168PA-328-328P_datasheet_Complete.pdf" rel="nofollow noreferrer">in the datasheet</a> as 2LSB. This means that the actual value could be anywhere between 728 and 732.</p>

<p>Of course this depends on the accuracy of V<sub>CC</sub>, since it is being used as the ADC reference. This is why supply voltage measurements should be performed <a href="https://arduino.stackexchange.com/questions/1794/arduino-adc-reference-voltage-if-it-is-battery-powered">using the internal bandgap voltage</a> instead.</p>
