Based on series of comparisons with the reference voltage, {IDF_TARGET_NAME} ADC determines each bit of the output digital result. Per design the {IDF_TARGET_NAME} ADC reference voltage is 1100 mV, however the true reference voltage can range from 1000 mV to 1200 mV among different chips. This guide will introduce an ADC calibration driver to minimize this difference.
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 - convers 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.
.. only:: esp32
- `Kconfig Options <#kconfig-options>`__ - lists the supported Kconfig options that can be used to make a different effect on driver behavior.
The ADC calibration driver provides ADC calibration scheme(s). From calibration driver's point of view, an ADC calibration scheme is created to an ADC calibration handle :cpp:type:`adc_cali_handle_t`.
:cpp:func:`adc_cali_check_scheme` can be used to know which calibration scheme is supported on the chip. For those users who are already aware of the supported scheme, this step can be skipped. Just call the corresponding function to create the scheme handle.
For those users who use their custom ADC calibration schemes, you could either modify this function :cpp:func:`adc_cali_check_scheme`, or just skip this step and call your custom creation function.
.. only:: esp32 or esp32s2 or esp32c2
ADC Calibration Line Fitting Scheme
```````````````````````````````````
{IDF_TARGET_NAME} supports :c:macro:`ADC_CALI_SCHEME_VER_LINE_FITTING` scheme. To create this scheme, set up :cpp:type:`adc_cali_line_fitting_config_t` first.
- :cpp:member:`adc_cali_line_fitting_config_t::unit_id`, the ADC that your ADC raw results are from.
- :cpp:member:`adc_cali_line_fitting_config_t::atten`, ADC attenuation that your ADC raw results use.
- :cpp:member:`adc_cali_line_fitting_config_t::bitwidth`, the ADC raw result bitwidth.
.. only:: esp32
There is also a configuration :cpp:member:`adc_cali_line_fitting_config_t::default_vref`. Normally this can be simply set to 0. Line Fitting scheme doesn't rely on this value. However, if the Line Fitting scheme required eFuse bits are not burnt on your board, driver will rely on this value to do the calibration.
You can use :cpp:func:`adc_cali_scheme_line_fitting_check_efuse` to check the eFuse bits. Normally the Line Fitting scheme eFuse value will be :c:macro:`ADC_CALI_LINE_FITTING_EFUSE_VAL_EFUSE_TP` or :c:macro:`ADC_CALI_LINE_FITTING_EFUSE_VAL_EFUSE_VREF`. This means Line Fitting scheme will use calibration parameters burnt in the eFuse to do the calibration.
When the Line Fitting scheme eFuse value is :c:macro:`ADC_CALI_LINE_FITTING_EFUSE_VAL_DEFAULT_VREF`, you need to set the :cpp:member:`esp_adc_cali_line_fitting_init::default_vref`. Default vref is an estimate of the ADC reference voltage provided by the users as a parameter during calibration.
After setting up the configuration structure, call :cpp:func:`adc_cali_create_scheme_line_fitting` to create a Line Fitting calibration scheme handle.
.. only:: esp32s2
This function may fail due to reasons such as :c:macro:`ESP_ERR_INVALID_ARG` or :c:macro:`ESP_ERR_NO_MEM`. Especially, when the function return :c:macro:`ESP_ERR_NOT_SUPPORTED`, this means the calibration scheme required eFuse bits are not burnt on your board.
.. code:: c
ESP_LOGI(TAG, "calibration scheme version is %s", "Line Fitting");
adc_cali_line_fitting_config_t cali_config = {
.unit_id = unit,
.atten = atten,
.bitwidth = ADC_BITWIDTH_DEFAULT,
};
ESP_ERROR_CHECK(adc_cali_create_scheme_line_fitting(&cali_config, &handle));
When the ADC calibration is no longer used, please delete the calibration scheme handle by calling :cpp:func:`adc_cali_delete_scheme_line_fitting`.
Delete Line Fitting Scheme
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. code:: c
ESP_LOGI(TAG, "delete %s calibration scheme", "Line Fitting");
ESP_ERROR_CHECK(adc_cali_delete_scheme_line_fitting(handle));
.. only:: esp32c3 or esp32s3
ADC Calibration Curve Fitting Scheme
````````````````````````````````````
{IDF_TARGET_NAME} supports :c:macro:`ADC_CALI_SCHEME_VER_CURVE_FITTING` scheme. To create this scheme, set up :cpp:type:`adc_cali_curve_fitting_config_t` first.
- :cpp:member:`adc_cali_curve_fitting_config_t::unit_id`, the ADC that your ADC raw results are from.
- :cpp:member:`adc_cali_curve_fitting_config_t::atten`, ADC attenuation that your ADC raw results use.
- :cpp:member:`adc_cali_curve_fitting_config_t::bitwidth`, the ADC raw result bitwidth.
After setting up the configuration structure, call :cpp:func:`adc_cali_create_scheme_curve_fitting` to create a Curve Fitting calibration scheme handle. This function may fail due to reasons such as :c:macro:`ESP_ERR_INVALID_ARG` or :c:macro:`ESP_ERR_NO_MEM`. Especially, when the function return :c:macro:`ESP_ERR_NOT_SUPPORTED`, this means the calibration scheme required eFuse bits are not burnt on your board.
Create Curve Fitting Scheme
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. code:: c
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 :cpp:func:`adc_cali_delete_scheme_curve_fitting`.
Delete Curve Fitting Scheme
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. code:: c
ESP_LOGI(TAG, "delete %s calibration scheme", "Curve Fitting");
ESP_ERROR_CHECK(adc_cali_delete_scheme_curve_fitting(handle));
.. only:: esp32h4
There is no supported calibration scheme yet.
Note
For users who want to use their 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.
After setting up the calibration characteristics, you can call :cpp:func:`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 invalid argument. Especailly, if this function returns :c:macro:`ESP_ERR_INVALID_STATE`, this means the calibration scheme isn't created. You need to create a calibration scheme handle, use :cpp:func:`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.
.. only:: esp32c2
.. note::
ADC calibration is only supported under :c:macro:`ADC_ATTEN_DB_0` and :c:macro:`ADC_ATTEN_DB_11`. Under :c:macro:`ADC_ATTEN_DB_0`, input voltage higher than 950 mV is not supported. Under :c:macro:`ADC_ATTEN_DB_11`, input voltage higher than 2800 mV is not supported.
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]);The factory function :cpp:func:`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 :cpp:type:`adc_cali_handle_t` as the first positional parameter are not thread safe, you should avoid calling them from multiple tasks.
.. only:: esp32
Kconfig Options
^^^^^^^^^^^^^^^
- :ref:`CONFIG_ADC_CAL_EFUSE_TP_ENABLE`, disable this to decrease the code size, if you are aware of the calibration eFuse value :cpp:type:`ADC_CALI_LINE_FITTING_EFUSE_VAL_EFUSE_TP` isn't this one.
- :ref:`CONFIG_ADC_CAL_EFUSE_VREF_ENABLE`, disable this to decrease the code size, if you are aware of the calibration eFuse value :cpp:type:`ADC_CALI_LINE_FITTING_EFUSE_VAL_EFUSE_VREF` isn't this one.
- :ref:`CONFIG_ADC_CAL_LUT_ENABLE`, disable this to decrease the code size, if you don't calibrate the ADC raw results under :c:macro:`ADC_ATTEN_DB_11`.
The {IDF_TARGET_NAME} ADC can be 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.
.. only:: esp32
.. figure:: ../../../_static/diagrams/adc/adc-noise-graph.jpg
:align: center
:alt: ADC noise mitigation
Graph illustrating noise mitigation using capacitor and multisampling of 64 samples.
.. include-build-file:: inc/adc_cali.inc
.. include-build-file:: inc/adc_cali_scheme.inc