Converting ADC value to voltage in Embedded C

Introduction

We convert ADC values to voltage to understand real-world signals like temperature or light in volts, which are easier to interpret.

Reading sensor data like temperature or light intensity from an ADC.

Displaying the measured voltage on a screen or sending it to another device.

Making decisions in a program based on voltage thresholds.

Calibrating sensors by comparing ADC readings to known voltages.

Syntax

Embedded C

float voltage = (adc_value * reference_voltage) / max_adc_value;

adc_value is the number read from the ADC (usually an integer).

reference_voltage is the maximum voltage the ADC can measure (like 3.3V or 5V).

max_adc_value is the maximum ADC value (e.g., 1023 for 10-bit ADC).

Examples

For a 10-bit ADC (max value 1023) with 3.3V reference.

Embedded C

float voltage = (adc_value * 3.3) / 1023;

For a 12-bit ADC (max value 4095) with 5V reference.

Embedded C

float voltage = (adc_value * 5.0) / 4095;

Sample Program

This program converts a 10-bit ADC value of 512 to voltage using a 3.3V reference and prints the result.

Embedded C

#include <stdio.h>

int main() {
    int adc_value = 512; // Example ADC reading
    float reference_voltage = 3.3; // ADC reference voltage in volts
    int max_adc_value = 1023; // For 10-bit ADC

    float voltage = (adc_value * reference_voltage) / max_adc_value;

    printf("ADC Value: %d\n", adc_value);
    printf("Voltage: %.2f V\n", voltage);

    return 0;
}

Important Notes

Make sure to use floating point numbers for accurate voltage calculation.

The max ADC value depends on the ADC resolution: for 10-bit it's 1023, for 12-bit it's 4095, etc.

Reference voltage must match the voltage used by your ADC hardware.

Summary

ADC values are numbers from 0 to max ADC value representing voltage levels.

Convert ADC value to voltage by multiplying by reference voltage and dividing by max ADC value.

This helps to understand sensor readings in real voltage units.

Practice

1. What is the main purpose of converting an ADC value to voltage in embedded C?

easy

A. To reset the ADC value to zero

B. To understand sensor readings in real voltage units

C. To change the ADC hardware settings

D. To increase the ADC resolution

Solution

1. Step 1: Understand ADC value meaning

   ADC values represent digital numbers corresponding to analog voltage levels.

2. Step 2: Purpose of conversion

   Converting ADC values to voltage helps interpret sensor data in actual voltage units.

3. Final Answer:

   To understand sensor readings in real voltage units -> Option B

4. Quick Check:

   ADC value to voltage conversion = real voltage [OK]

Hint: ADC value maps to voltage for real-world meaning [OK]

Common Mistakes:

• Thinking conversion changes ADC resolution
• Confusing conversion with hardware setup
• Assuming conversion resets ADC values

2. Which of the following C code snippets correctly converts a 10-bit ADC value to voltage assuming a 5V reference?

easy

A. float voltage = (adc_value * 5.0) / 1023;

B. float voltage = (adc_value / 5.0) * 1023;

C. float voltage = 5.0 / (adc_value * 1023);

D. float voltage = (1023 / adc_value) * 5.0;

Solution

1. Step 1: Recall formula for voltage conversion

   Voltage = (ADC value * Reference voltage) / Max ADC value.

2. Step 2: Apply values for 10-bit ADC and 5V reference

   Max ADC value = 1023, Reference voltage = 5.0V, so voltage = (adc_value * 5.0) / 1023.

3. Final Answer:

   float voltage = (adc_value * 5.0) / 1023; -> Option A

4. Quick Check:

   Voltage formula matches float voltage = (adc_value * 5.0) / 1023; [OK]

Hint: Multiply by ref voltage, divide by max ADC value [OK]

Common Mistakes:

• Dividing adc_value by reference voltage instead
• Swapping numerator and denominator
• Using integer division without float casting

3. Given the code snippet:

int adc_value = 512;
float ref_voltage = 3.3;
float voltage = (adc_value * ref_voltage) / 1023;

What is the value of voltage approximately?

medium

A. 0.5

B. 3.3

C. 1.65

D. 2.5

Solution

1. Step 1: Substitute values into formula

   voltage = (512 * 3.3) / 1023.

2. Step 2: Calculate voltage

   512 * 3.3 = 1689.6; 1689.6 / 1023 ≈ 1.651.

3. Final Answer:

   1.65 -> Option C

4. Quick Check:

   Half of 3.3V ≈ 1.65V [OK]

Hint: 512 is about half of 1023, so voltage ~ half ref voltage [OK]

Common Mistakes:

• Using 1024 instead of 1023 as max ADC value
• Forgetting to use float for calculation
• Rounding errors without explanation

4. Identify the error in this code snippet for converting ADC value to voltage:

int adc_value = 700;
float ref_voltage = 5.0;
float voltage = adc_value / 1023 * ref_voltage;

medium

A. Missing semicolon after voltage calculation

B. Reference voltage should be an integer

C. ADC value must be float type

D. Integer division causes incorrect voltage calculation

Solution

1. Step 1: Analyze division operation

   adc_value and 1023 are integers, so adc_value / 1023 does integer division, truncating decimals.

2. Step 2: Effect on voltage calculation

   Integer division results in 0 for adc_value < 1023, so voltage becomes 0 * ref_voltage = 0 incorrectly.

3. Final Answer:

   Integer division causes incorrect voltage calculation -> Option D

4. Quick Check:

   Use float division to avoid zero result [OK]

Hint: Cast to float before division to fix [OK]

Common Mistakes:

• Assuming integer division works for float results
• Changing reference voltage type unnecessarily
• Ignoring missing semicolon which is present

5. You have a 12-bit ADC (max value 4095) and a 3.3V reference. Write the correct formula in C to convert an ADC reading stored in adc_val to voltage stored in voltage. Which option is correct?

hard

A. voltage = ((float)adc_val * 3.3) / 4095;

B. voltage = (adc_val / 4095) * 3.3;

C. voltage = adc_val / 4095 * 3.3;

D. voltage = adc_val * (3.3 / 4096);

Solution

1. Step 1: Understand 12-bit ADC max value and reference voltage

   Max ADC value is 4095, reference voltage is 3.3V.

2. Step 2: Ensure float division to avoid integer truncation

   Casting adc_val to float ensures correct floating-point division.

3. Step 3: Write formula with float cast

   voltage = ((float)adc_val * 3.3) / 4095; correctly converts ADC to voltage.

4. Final Answer:

   voltage = ((float)adc_val * 3.3) / 4095; -> Option A

5. Quick Check:

   Float cast prevents integer division errors [OK]

Hint: Cast ADC value to float before division [OK]

Common Mistakes:

• Omitting float cast causing zero voltage
• Swapping multiplication and division order
• Using 4096 instead of 4095 as max ADC value