Voltage sensor Calibration We had used a voltage sensor unit to measure the voltage of the odroid board using an Arduino Uno. wWhen I uploaded the sketch and got the output, the value which was being displayed was around 4.7 to 4.9V, but when I measured the top of the voltage sensor module using a multimeter, it had a stable voltage of 5.08V. Is their any particular reason why this is happening?

Voltage sensor module

int analogInput = A0;
float vout = 0.0;
float vin = 0.0;
float R1 = 30000.0; 
float R2 = 7500.0; 
int value = 0;
void setup() {  
pinMode(analogInput, INPUT);
  Serial.begin(9600);
  Serial.print("DC VOLTMETER");
}
void loop() {
  // read the value at analog input  
value = analogRead(analogInput);  
vout = (value * 5.0) / 1024.0; 
vin = vout / (R2 / (R1 + R2));  
Serial.print("INPUT V= ");  
Serial.println(vin, 2);  
delay(500);
}
Multimeters are designed to provide a stable response. It would be averaging the readings. 

As far as the difference in voltage reading, if you are using the Arduino board as is the default is to assume a reference voltage for the A/D as being 5.0 volts.(for your calculations) That may not be the case. Also there may be a tolerance issue on the measurement board, but I think the reference for the A/D on the Arduino is the bigger issue.