├── ESP32_ADC_Read_Voltage_Accuracy_V2.ino
├── ESP32_ADC_VREF.ino
└── README.md


/ESP32_ADC_Read_Voltage_Accuracy_V2.ino:
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 1 | #include "esp_adc_cal.h"
 2 | 
 3 | #define ADC_PIN     35               // Set for the Lolin D32 board with on-board voltage divider
 4 | #define voltage_divider_offset 2.174 // Should be a value of 2.000, but ADC input impedance loads the voltage divider, requiring a correction
 5 | 
 6 | void setup() {
 7 |   Serial.begin(115200);
 8 | }
 9 | 
10 | void loop() {
11 |   float unadjusted_voltage = (analogRead(ADC_PIN) / 4095.0 * 3.3 * voltage_divider_offset);
12 |   float adjusted_voltage   = ReadVoltage(ADC_PIN);
13 |   Serial.println("Un-adjusted voltage = " + String(unadjusted_voltage,3) + "v");
14 |   Serial.println("   Adjusted voltage = " + String(adjusted_voltage,3) + "v");
15 |   Serial.println();
16 |   delay(5000);
17 | }
18 | 
19 | float ReadVoltage(byte ADC_Pin) {
20 |   float calibration  = 1.000; // Adjust for ultimate accuracy when input is measured using an accurate DVM, if reading too high then use e.g. 0.99, too low use 1.01
21 |   float vref = 1100;
22 |   esp_adc_cal_characteristics_t adc_chars;
23 |   esp_adc_cal_characterize(ADC_UNIT_1, ADC_ATTEN_DB_11, ADC_WIDTH_BIT_12, 1100, &adc_chars);
24 |   vref = adc_chars.vref; // Obtain the device ADC reference voltage
25 |   return (analogRead(ADC_Pin) / 4095.0) * 3.3 * voltage_divider_offset * (1100 / vref) * calibration;  // ESP by design reference voltage in mV
26 | }
27 | 
28 | // The esp_adc_cal/include/esp_adc_cal.h API provides functions to correct for differences
29 | // in measured voltages caused by variation of ADC reference voltages (Vref) between chips.
30 | // Per design the ADC reference voltage is 1100 mV, however the true reference voltage can
31 | // range from 1000 mV to 1200 mV amongst different ESP32's
32 | 


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/ESP32_ADC_VREF.ino:
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 1 | #include "esp_adc_cal.h"
 2 | 
 3 | #define ADC_PIN     36
 4 | 
 5 | void setup() {
 6 |   Serial.begin(115200);
 7 | }
 8 | 
 9 | void loop() {
10 |   float unadjusted_voltage = (analogRead(ADC_PIN) / 4095.0 * 3.3);
11 |   float adjusted_voltage   = ReadVoltage(ADC_PIN);
12 |   
13 |   Serial.println("      Input Voltage = 3.000 v");
14 |   Serial.println("------------------------------");
15 |   Serial.println("   Adjusted Voltage = " + String(adjusted_voltage, 3) + "v  " + String(adjusted_voltage / 3.00 * 100 - 100) + "% error");
16 |   Serial.println("Un-adjusted Voltage = " + String(unadjusted_voltage, 3) + "v  " + String(unadjusted_voltage / 3.00 * 100 - 100) + "% error");
17 |   Serial.println();
18 |   delay(5000);
19 | }
20 | 
21 | float ReadVoltage(byte ADC_Pin) {
22 |   float calibration  = 1.000; // Adjust for ultimate accuracy when input is measured using an accurate DVM, if reading too high then use e.g. 0.99, too low use 1.01
23 |   float vref = 1100;
24 |   esp_adc_cal_characteristics_t adc_chars;
25 |   esp_adc_cal_characterize(ADC_UNIT_1, ADC_ATTEN_DB_11, ADC_WIDTH_BIT_12, 1100, &adc_chars);
26 |   vref = adc_chars.vref; // Obtain the device ADC reference voltage
27 |   return (analogRead(ADC_Pin) / 4095.0) * 3.3 * (1100 / vref) * calibration;  // ESP by design reference voltage in mV
28 | }
29 | 
30 | // The esp_adc_cal/include/esp_adc_cal.h API provides functions to correct for differences
31 | // in measured voltages caused by variation of ADC reference voltages (Vref) between chips.
32 | // Per design the ADC reference voltage is 1100 mV, however the true reference voltage can
33 | // range from 1000 mV to 1200 mV amongst different ESP32's
34 | 


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/README.md:
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 1 | # ESP32-ADC-Accuracy-Improvement
 2 | Extracting the ESP32 ADC reference voltage to improve ADC reading accuracy
 3 | 
 4 | The 'esp_adc_cal/include/esp_adc_cal.h' API provides a function to correct for differences
 5 | in measured voltages caused by variation of ADC reference voltages (Vref) between chips.
 6 | 
 7 | By-design the ADC reference voltage is 1100 mV, however the true reference voltage can
 8 | vary from device-to-deice and range from 1000 mV to 1200 mV amongst different ESP32's.
 9 | 
10 | Reading the reference voltage enables a correction to be made which then improves ADC reading accuracy. 
11 | 
12 | NOTE:
13 | 
14 | For most boards with an on-board voltage divder of 2 series 220K resistors, the divide by 2 factor needs to be adjusted to 2.174 to allow for ADC input impedance loading the voltage divider.
15 | 
16 | The V2 code makes that correction.
17 | 


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