Arduino Mega 2560 reading the same value in all analog inputs

Development Tools IDE 1.x
1

I have an Arduino Mega 2560, suddenly its analog inputs (A0 to A15) started reading the same value all the time without anything connected. The values of ADC output in ANY analog input are around 350~400 (decimal)
If I connect some signal (for exemple, in normal conditions an input around 1V, it would give around 200 (DEC) in the ADC output, right?) but it sums this 300 error value in output with this 200, resulting 500

The point is: It's reading always the same value and sum this value to any signal inputed

Example code used:

int sensorPin = A1; // or A0, A2, A3... all of them
int sensorValue=0;

void setup() {
Serial.begin(9600);

}

void loop() {
sensorValue = analogRead (sensorPin);
Serial.print ("Sensor = ");
Serial.println (sensorValue,DEC);
delay(300);
}

no inputs.jpg

1 v.jpg

2

Mutuca_42:
its analog inputs (A0 to A15) started reading the same value all the time without anything connected.

There is only one Sample/Hold capacitor in the input of the A/D converter. If there is nothing connected to your analog input pin there is nothing to sink or source current and that capacitor will retain the last voltage put on it. Your hardware is acting exactly as one would expect.
You can NEVER rely on any particular value when reading an unconnected pin. Use a pull-up or pull-down resistor if you want a pin to read HIGH/1023 or LOW/0 when nothing else is connected.

3

Make a small test sketch, and try the 1V again. Also connect an analog input to GND and 5V and 3.3V.

The open inputs doesn't say anything as johnwasser wrote.
The 1V could be 500 if you have the internal reference of 2.56V in your code.

If the values are really wrong, then something might be broken. It is possible that a part of the analog section inside the chip is damaged.

Have you protected the analog inputs with resistors ? or is it possible that a voltage peak from a long wire or a motor goes directly into an analog input ?