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### Topic: ADC accuracy problem (Read 103 times)previous topic - next topic

#### SupArdu ##### May 07, 2019, 09:05 pm
Hi,
after having some problems measuring analog voltages with an Arduino Nano in my application I made a test setup:
An Arduino Nano, powered by the USB connection, is continuously reading an analog voltage and outputting the value read on Serial Monitor.

Curcuit:
- the output of a reference voltage chip is connected to Vref of the Nano; reference voltage = 2.46 V
- a potentiometer between Gnd and 5V output of the Nano provides a variable voltage to A3.
- a voltmeter is connected to A3

Code:
Code: [Select]
`int V_batt;int i;setup {  analogReference(EXTERNAL);  while (1) {    V_batt=0;    for (i=0; i<16; i++) {      V_bat += analogRead(A3);      delay(1);    }    V_batt = (V_batt + 8)>>4;    Serial.print("V=");    Serial.println(V_batt);    delay(500);  }}`
With this simple test setup I varied the analog input via the potentiometer and made a table with several different voltages at A3 and the values of V_batt on the Serial Monitor.

With the reference voltage of Vref=2.46 V the formula V_A3*1023/V_batt=Vref should always verify that Vref=2.46. But this never is true! My measurements are resulting in varying values of Vref=2.48 ... 2.82 V (mean value 2.7258).

The values are varying because the voltmeter only has a resolution of 10 mV. That's ok.
But I cannot find an explanation for the high mean value of computed Vref. The ADC seems to see 2.7258 V at the Vref pin instead of 2.46 V.

Has anybody an explanation?

-------------------
have success
SupArdu

#### PaulS #1
##### May 08, 2019, 12:28 pm
Quote
But I cannot find an explanation for the high mean value of computed Vref.
Since you didn't show us how you arrived at that value, you can't expect us to explain it.
The art of getting good answers lies in asking good questions.

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