Calibrating Arduino's ADC with Internal Reference

Improving Analog-to-Digital Converter (ADC) Accuracy in Arduino

In this article, we will explore a technique to enhance the accuracy of the built-in analog-to-digital converter (ADC) in Arduino boards. The ADC is a crucial component in many projects, as it allows us to read analog values from sensors and other devices.

The Problem with Arduino's Built-in ADC

Arduino boards have a built-in 10-bit ADC, which can be used to read analog values on pins A0 to A7. However, the accuracy of the ADC is dependent on the reference voltage, which in Arduino's case is the supply voltage (Vcc). Unfortunately, Vcc can vary depending on the power source and other factors, leading to inaccurate readings from the ADC.

Demonstrating the Problem

To illustrate this issue, we can supply an Arduino with a variable Vcc and measure a fixed voltage of 3 volts using the ADC. The measured value will be displayed on a TFT screen. By varying the Vcc between 4.8V and 5.2V, we can see how the measured value changes.

The Solution: Using the Internal Reference Voltage

Fortunately, Arduino boards have a built-in internal reference voltage of 1.1V, which can be used to improve the accuracy of the ADC. By measuring this internal reference voltage with the ADC and using it as a calibration point, we can calculate the actual Vcc.

The Formula for Calculating Vcc

Vcc = (1.1V x 1023 x 1000) / measured_result

Verifying the Solution

We can verify that this solution works by measuring Vcc using the internal reference voltage and comparing it to the actual Vcc. By varying Vcc between 4V and 5.3V, we can see that the measured value remains accurate.

Correcting the Initial Measurement

Now that we have an accurate Vcc, we can use it to correct our initial measurement. The formula for this is:

corrected_measurement = (Vcc / 5) x measured_value

Code Example

// Code to measure supply voltage
// ...

This code uses bit-banging and works on most Arduino boards.

Conclusion

In this article, we explored a technique to improve the accuracy of the built-in ADC in Arduino boards. By using the internal reference voltage and calculating Vcc, we can achieve more accurate readings from the ADC.

Calibrating Arduino's ADC with Internal Reference

Arduino boards are equipped with a built-in Analog-to-Digital Converter (ADC) that enables them to read analog signals from sensors and other external sources. However, the accuracy of these readings can be affected by various factors such as temperature changes, power supply fluctuations, and component tolerances. To mitigate these effects, it is possible to calibrate the ADC using an internal reference voltage. In this article, we will explore the process of calibrating Arduino's ADC with internal reference.

Understanding the Internal Reference Voltage

The internal reference voltage is a stable voltage source that is generated within the Arduino board. This voltage is used as a reference point for the ADC conversions, allowing for more accurate readings. The internal reference voltage is typically around 1.1V, but it can vary slightly from one board to another.

Calibration Process

1. Measure the Internal Reference Voltage: Before calibration, it is essential to measure the internal reference voltage of your Arduino board. This can be done using a multimeter or an oscilloscope.
2. Run the Calibration Sketch: The next step is to run a calibration sketch on your Arduino board. This sketch will read the internal reference voltage and store it in a variable.
3. Calculate the Correction Factor: After running the calibration sketch, you need to calculate the correction factor by dividing the measured internal reference voltage by the nominal value (1.1V).
4. Apply the Correction Factor: Finally, apply the calculated correction factor to your ADC readings to obtain accurate results.

Example Code

// Calibration sketch for Arduino's ADC with internal reference

const int INTERNAL_REFERENCE_VOLTAGE = 1100; // mV
const int ANALOG_PIN = A0;

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

void loop() {
  int adcReading = analogRead(ANALOG_PIN);
  float measuredVoltage = (adcReading * INTERNAL_REFERENCE_VOLTAGE) / 1024.0;
  Serial.print("Measured Voltage: ");
  Serial.println(measuredVoltage);

  // Calculate correction factor
  float correctionFactor = measuredVoltage / INTERNAL_REFERENCE_VOLTAGE;

  // Apply correction factor to ADC readings
  int correctedAdcReading = (adcReading * correctionFactor);
  Serial.print("Corrected ADC Reading: ");
  Serial.println(correctedAdcReading);

  delay(1000);
}

Conclusion

Calibrating Arduino's ADC with internal reference is a straightforward process that can significantly improve the accuracy of your analog readings. By following these steps and using the provided example code, you can ensure that your projects produce reliable and consistent results.

1. Connect the analog input pin to the internal reference voltage source.
2. Set the ADC to use the internal reference voltage as the reference source.
3. Take multiple readings of the internal reference voltage using the ADC.
4. Calculate the average value of the readings.
5. Store the average value in a variable or EEPROM for future use.