What could be the cause of a constant voltage offset on the STM32L041G6 ADC?

Question

I have been testing the ADC on my STM32L0 with 12-bit resolution and for some reason, every measurement I take has a 0.048 V offset. I've tried a different supply voltage (from 1.8 V to 3.3 V), but the offset stays there.

This offset starts at the first count, when I apply 0 V to the ADC input and read 130 counts (with 1.8 V Vdda) it stays for the whole range.

My voltage source is very stable and I've verified my ADC input voltage and the STM32 supply voltage with 3 different accurate pieces of measuring equipment.

The board I'm using is a custom PCB, but I only placed the STM32 and some passive components to get it to function (coupling capacitors and reset pull-up).

I also tested multiple boards with different configurations and I always get this offset voltage. I outputed Vrefint and measured it, and I got 1.225 V (specified is 1.224 V) so the ADC reference seems fine too.

I was wondering if someone has a suggestion on the possible cause of this offset error? I've been looking for a solution but I'm having no luck so far, so I would really appreciate the input of more experienced developers.

Here is a simplified version of the code I use :

    /* USER CODE BEGIN Header */
/**
  ******************************************************************************
  * @file    adc.c
  * @brief   This file provides code for the configuration
  *          of the ADC instances.
  ******************************************************************************
  * @attention
  *
  * Copyright (c) 2022 STMicroelectronics.
  * All rights reserved.
  *
  * This software is licensed under terms that can be found in the LICENSE file
  * in the root directory of this software component.
  * If no LICENSE file comes with this software, it is provided AS-IS.
  *
  ******************************************************************************
  */
/* USER CODE END Header */
/* Includes ------------------------------------------------------------------*/
#include "adc.h"

/* USER CODE BEGIN 0 */

/* USER CODE END 0 */

ADC_HandleTypeDef hadc;

/* ADC init function */
void MX_ADC_Init(void)
{

  /* USER CODE BEGIN ADC_Init 0 */

  /* USER CODE END ADC_Init 0 */

  ADC_ChannelConfTypeDef sConfig = {0};

  /* USER CODE BEGIN ADC_Init 1 */

  /* USER CODE END ADC_Init 1 */

  /** Configure the global features of the ADC (Clock, Resolution, Data Alignment and number of conversion)
  */
  hadc.Instance = ADC1;
  hadc.Init.OversamplingMode = DISABLE;
  hadc.Init.ClockPrescaler = ADC_CLOCK_SYNC_PCLK_DIV1;
  hadc.Init.Resolution = ADC_RESOLUTION_12B;
  hadc.Init.SamplingTime = ADC_SAMPLETIME_1CYCLE_5;
  hadc.Init.ScanConvMode = ADC_SCAN_DIRECTION_FORWARD;
  hadc.Init.DataAlign = ADC_DATAALIGN_RIGHT;
  hadc.Init.ContinuousConvMode = DISABLE;
  hadc.Init.DiscontinuousConvMode = DISABLE;
  hadc.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;
  hadc.Init.ExternalTrigConv = ADC_SOFTWARE_START;
  hadc.Init.DMAContinuousRequests = DISABLE;
  hadc.Init.EOCSelection = ADC_EOC_SINGLE_CONV;
  hadc.Init.Overrun = ADC_OVR_DATA_PRESERVED;
  hadc.Init.LowPowerAutoWait = DISABLE;
  hadc.Init.LowPowerFrequencyMode = ENABLE;
  hadc.Init.LowPowerAutoPowerOff = DISABLE;
  if (HAL_ADC_Init(&hadc) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure for the selected ADC regular channel to be converted.
  */
  sConfig.Channel = ADC_CHANNEL_0;
  sConfig.Rank = ADC_RANK_CHANNEL_NUMBER;
  if (HAL_ADC_ConfigChannel(&hadc, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN ADC_Init 2 */

  /* USER CODE END ADC_Init 2 */

}

void HAL_ADC_MspInit(ADC_HandleTypeDef* adcHandle)
{

  GPIO_InitTypeDef GPIO_InitStruct = {0};
  if(adcHandle->Instance==ADC1)
  {
  /* USER CODE BEGIN ADC1_MspInit 0 */

  /* USER CODE END ADC1_MspInit 0 */
    /* ADC1 clock enable */
    __HAL_RCC_ADC1_CLK_ENABLE();

    __HAL_RCC_GPIOA_CLK_ENABLE();
    /**ADC GPIO Configuration
    PA0-CK_IN     ------> ADC_IN0
    */
    GPIO_InitStruct.Pin = SENSOR_ANALOG_IN_Pin;
    GPIO_InitStruct.Mode = GPIO_MODE_ANALOG;
    GPIO_InitStruct.Pull = GPIO_NOPULL;
    HAL_GPIO_Init(SENSOR_ANALOG_IN_GPIO_Port, &GPIO_InitStruct);

  /* USER CODE BEGIN ADC1_MspInit 1 */

  /* USER CODE END ADC1_MspInit 1 */
  }
}

void HAL_ADC_MspDeInit(ADC_HandleTypeDef* adcHandle)
{

  if(adcHandle->Instance==ADC1)
  {
  /* USER CODE BEGIN ADC1_MspDeInit 0 */

  /* USER CODE END ADC1_MspDeInit 0 */
    /* Peripheral clock disable */
    __HAL_RCC_ADC1_CLK_DISABLE();

    /**ADC GPIO Configuration
    PA0-CK_IN     ------> ADC_IN0
    */

  /* USER CODE BEGIN ADC1_MspDeInit 1 */

  /* USER CODE END ADC1_MspDeInit 1 */
  }
}

/* USER CODE BEGIN 1 */

/* USER CODE END 1 */

And here is the main file where I call the function to read the adc and send it to uart to be read :

#include "main.h"

#include "gpio.h"
#include "adc.h"
#include "usart.h"

#include <stdlib.h>

extern "C" 

void execute(){
   
   HAL_Delay(10);
   HAL_ADC_Start(&hadc);
   HAL_ADC_PollForConversion(&hadc, HAL_MAX_DELAY);
   const auto value = HAL_ADC_GetValue(&hadc);
   char buffer [5];
   itoa(value, buffer, 10);
   HAL_UART_Transmit(&huart2, reinterpret_cast<uint8_t*>(buffer), 5, HAL_MAX_DELAY);
}

Answer 1

I found out what the issue was, my adc wasn't initialized and that was the cause of my offset, to solve the issue, I simply added this code in initialize() :

    void initialize()
{
    HAL_ADCEx_Calibration_Start(&hadc, ADC_SINGLE_ENDED);
    HAL_Delay(10);
}

My adc precision now is about 3 count (1.2mV) which is really good.