Analog to Digital Converter (ADC) Calibration Driver

Introduction

In ESP32-C6, the digital-to-analog converter (ADC) compares the input analog voltage to the reference, and determines each bit of the output digital result. By design, the ADC reference voltage for ESP32-C6 is 1100 mV. However, the true reference voltage can range from 1000 mV to 1200 mV among different chips. This guide introduces the ADC calibration driver to minimize the effect of different reference voltages, and get more accurate output results.

Functional Overview

The following sections of this document cover the typical steps to install and use the ADC calibration driver:

Calibration Scheme Creation - covers how to create a calibration scheme handle and delete the calibration scheme handle.
Result Conversion - covers how to convert ADC raw result to calibrated result.
Thread Safety - lists which APIs are guaranteed to be thread-safe by the driver.
Minimize Noise - describes a general way to minimize the noise.

Calibration Scheme Creation

The ADC calibration driver provides ADC calibration scheme(s). From the calibration driver's point of view, an ADC calibration scheme is created for an ADC calibration handle adc_cali_handle_t.

adc_cali_check_scheme() can be used to know which calibration scheme is supported on the chip. If you already know the supported schemes, this step can be skipped. Just call the corresponding function to create the scheme handle.

If you use your custom ADC calibration schemes, you could either modify this function adc_cali_check_scheme(), or just skip this step and call your custom creation function.

ADC Calibration Curve Fitting Scheme

ESP32-C6 supports ADC_CALI_SCHEME_VER_CURVE_FITTING scheme. To create this scheme, set up adc_cali_curve_fitting_config_t first.

adc_cali_curve_fitting_config_t::unit_id, the ADC that your ADC raw results are from.
adc_cali_curve_fitting_config_t::chan, the ADC channel that your ADC raw results are from. The calibration scheme not only differs by attenuation but is also related to the channels.
adc_cali_curve_fitting_config_t::atten, ADC attenuation that your ADC raw results use.
adc_cali_curve_fitting_config_t::bitwidth, bit width of ADC raw result.

After setting up the configuration structure, call adc_cali_create_scheme_curve_fitting() to create a Curve Fitting calibration scheme handle. This function may fail due to reasons such as ESP_ERR_INVALID_ARG or ESP_ERR_NO_MEM. Especially, when the function return ESP_ERR_NOT_SUPPORTED, this means the calibration scheme required eFuse bits are not burnt on your board.

Create Curve Fitting Scheme

ESP_LOGI(TAG, "calibration scheme version is %s", "Curve Fitting");
adc_cali_curve_fitting_config_t cali_config = {
    .unit_id = unit,
    .atten = atten,
    .bitwidth = ADC_BITWIDTH_DEFAULT,
};
ESP_ERROR_CHECK(adc_cali_create_scheme_curve_fitting(&cali_config, &handle));

When the ADC calibration is no longer used, please delete the calibration scheme driver from the calibration handle by calling adc_cali_delete_scheme_curve_fitting().

Delete Curve Fitting Scheme

ESP_LOGI(TAG, "delete %s calibration scheme", "Curve Fitting");
ESP_ERROR_CHECK(adc_cali_delete_scheme_curve_fitting(handle));

Note

If you want to use your custom calibration schemes, you could provide a creation function to create your calibration scheme handle. Check the function table adc_cali_scheme_t in components/esp_adc/interface/adc_cali_interface.h to know the ESP ADC calibration interface.

Result Conversion

After setting up the calibration characteristics, you can call adc_cali_raw_to_voltage() to convert the ADC raw result into calibrated result. The calibrated result is in the unit of mV. This function may fail due to an invalid argument. Especially, if this function returns ESP_ERR_INVALID_STATE, this means the calibration scheme is not created. You need to create a calibration scheme handle, use adc_cali_check_scheme() to know the supported calibration scheme. On the other hand, you could also provide a custom calibration scheme and create the handle.

Get Voltage

ESP_ERROR_CHECK(adc_cali_raw_to_voltage(adc_cali_handle, adc_raw[0][0], &voltage[0][0]));
ESP_LOGI(TAG, "ADC%d Channel[%d] Cali Voltage: %d mV", ADC_UNIT_1 + 1, EXAMPLE_ADC1_CHAN0, voltage[0][0]);

Thread Safety

The factory function esp_adc_cali_new_scheme() is guaranteed to be thread-safe by the driver. Therefore, you can call them from different RTOS tasks without protection by extra locks.

Other functions that take the adc_cali_handle_t as the first positional parameter are not thread-safe, you should avoid calling them from multiple tasks.

Minimize Noise

The ESP32-C6 ADC is sensitive to noise, leading to large discrepancies in ADC readings. Depending on the usage scenario, you may need to connect a bypass capacitor (e.g., a 100 nF ceramic capacitor) to the ADC input pad in use, to minimize noise. Besides, multisampling may also be used to further mitigate the effects of noise.

API Reference

Header File

components/esp_adc/include/esp_adc/adc_cali.h
This header file can be included with:
#include "esp_adc/adc_cali.h"
This header file is a part of the API provided by the esp_adc component. To declare that your component depends on esp_adc, add the following to your CMakeLists.txt:
REQUIRES esp_adc
or
PRIV_REQUIRES esp_adc

Functions

esp_err_t adc_cali_check_scheme(adc_cali_scheme_ver_t *scheme_mask)

Check the supported ADC calibration scheme.

Parameters

scheme_mask -- [out] Supported ADC calibration scheme(s)

Returns

ESP_OK: On success
ESP_ERR_INVALID_ARG: Invalid argument
ESP_ERR_NOT_SUPPORTED: No supported calibration scheme

esp_err_t adc_cali_raw_to_voltage(adc_cali_handle_t handle, int raw, int *voltage)

Convert ADC raw data to calibrated voltage.

Parameters

handle -- [in] ADC calibration handle
raw -- [in] ADC raw data
voltage -- [out] Calibrated ADC voltage (in mV)

Returns

ESP_OK: On success
ESP_ERR_INVALID_ARG: Invalid argument
ESP_ERR_INVALID_STATE: Invalid state, scheme didn't registered

Type Definitions

typedef struct adc_cali_scheme_t *adc_cali_handle_t: ADC calibration handle.

Enumerations

enum adc_cali_scheme_ver_t

ADC calibration scheme.

Values:

enumerator ADC_CALI_SCHEME_VER_LINE_FITTING: Line fitting scheme.

enumerator ADC_CALI_SCHEME_VER_CURVE_FITTING: Curve fitting scheme.

Header File

components/esp_adc/include/esp_adc/adc_cali_scheme.h
This header file can be included with:
#include "esp_adc/adc_cali_scheme.h"
This header file is a part of the API provided by the esp_adc component. To declare that your component depends on esp_adc, add the following to your CMakeLists.txt:
REQUIRES esp_adc
or
PRIV_REQUIRES esp_adc

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