How to use Hardware Oversampling in STM32 ADC

ADC oversampling is a powerful technique that allows STM32 microcontrollers to extract more useful information from analog signals without adding extra hardware. By intelligently combining multiple ADC samples, oversampling can significantly reduce noise and improve the effective resolution of measurements.

In this tutorial, we explore how ADC oversampling works in STM32 MCUs, how it is implemented using CubeMX, and what impact it has on conversion time and sampling rate. With practical configuration steps and real output observations, you’ll see when oversampling makes sense and how to use it correctly in your applications.

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This is the 8^th tutorial in the STM32 ADC series. In the previous tutorials we covered how to configure the ADC in STM32 F1, F4 and F7 series and how to use it in the single channel polling, interrupt and DMA modes to read the potentiometer data. We have also covered the Multiple channels in DMA Normal Mode, Circular Mode and Multiple Channels with DMA.

You Do not need to go through the previous tutorials in order to understand this one.

What Is ADC Oversampling and Why Do We Need It?

ADC oversampling is a technique where the ADC samples the same analog signal multiple times and then combines those samples to produce a single output value. Instead of relying on one conversion per result, the ADC accumulates several conversions internally and optionally right-shifts the result to scale it back into a usable range.

The primary reason we use oversampling is to improve measurement quality without changing hardware. Real-world analog signals always contain noise, and a single ADC conversion can fluctuate by a few LSBs even when the input voltage is stable. By oversampling, this random noise tends to average out, resulting in a smoother and more stable ADC output.

Another important benefit is increased effective resolution. With proper oversampling and right shifting, a 12-bit ADC can effectively behave like a 14-bit or even 16-bit ADC. This is extremely useful in applications such as sensor readings, battery voltage measurement, or low-level analog signals where small changes matter.

However, oversampling is not free. Since multiple conversions are required for one final result, the effective sampling rate is reduced, and the total conversion time increases. This makes oversampling ideal for slow-changing signals, but less suitable for high-speed measurements.

Below is a simple diagram showing how multiple ADC samples are accumulated into a single oversampled result.

How Oversampling Improves ADC Resolution (Math)

In STM32 ADCs, oversampling works by accumulating multiple ADC conversions and then optionally right-shifting the result. The increase in effective resolution depends on how many samples are accumulated.

The relationship between oversampling ratio and resolution improvement is given by:

$$\text{Extra Bits} = \frac{1}{2} \log_2(N)$$

Where:

• $N$N = number of samples accumulated (oversampling ratio)

Example 1: 4× Oversampling

$$\frac{1}{2} \log_2(4) = \frac{1}{2} \times 2 = 1 \text{ extra bit}$$

A 12-bit ADC behaves like a 13-bit ADC.

Example 2: 16× Oversampling

$$\frac{1}{2} \log_2(16) = \frac{1}{2} \times 4 = 2 \text{ extra bits}$$

A 12-bit ADC behaves like a 14-bit ADC.

Example 3: 256× Oversampling

$$\frac{1}{2} \log_2(256) = \frac{1}{2} \times 8 = 4 \text{ extra bits}$$

A 12-bit ADC behaves like a 16-bit ADC.

Right Shift and Final ADC Value

After accumulation, the ADC result is typically right-shifted to bring the value back into a usable range:

$$\text{Final ADC Value} = \frac{\sum \text{Samples}}{2^{\text{Right Shift}}}$$

In CubeMX:

• Oversampling Ratio = number of samples accumulated
• Right Bit Shift = scaling factor to match the desired resolution

For example:

• Oversampling ratio = 16
• Right shift = 2 bits

This configuration yields a 14-bit effective result from a 12-bit ADC while keeping the output properly scaled.

STM32CubeMX Configuration

Before diving into the code, we need to configure the ADC and related peripherals using STM32CubeMX. In this section, we’ll go through the essential CubeMX settings required to make ADC oversampling work as expected.

Clock Configuration

Below is the image showing the clock configuration for STM32L496ZG.

I have configured the system clock to run at maximum 80MHz. The ADC is also configured to run at 64MHz.

ADC Configuration

Below is the image showing the ADC configuration.

• I have configured the ADC1 CH8, which is connected to pin PA3.
• The clock prescaler is set to 1, so the ADC clock is still at 64MHz (64MHz/PSC).
• The Resolution is set to 12-bit. The extra samples we take, will be accumulated on 12-bit.
• The continuous conversion mode is enabled, so the next conversion will automatically start once the previous conversion is over.
• The DMA continuous request is enabled, so the DMA will fetch the data from the channel continuously.

ADC Oversampling configuration

Below is the image showing the oversampling and channel configuration of the ADC.

Here we will first enable the oversampling for the Regular channels. As I mentioned, below is the formula to calculate the oversampling bit width.

Here I am choosing the OSR as 64. Therefore 64 samples will be collected for each trigger (software start). The Right shift (M) is set to 2. This will result in the bit width of 16 bits.

As I already mentioned, the STM32L496 has 16-bit ADC Data register, therefore we can not have the bit width higher than this value. Therefore the bit shift (M) and OSR should be selected in a way that the bit width is not higher than 16 bits.

The sampling time for the Channel 8 is set to 47.5 Cycles. With the ADC clock of 64 megahertz, the total time to convert the channel should be as shown below.

• Here 12.5 is the ADC clock for 12-bit Resolution.
• 47.5 is the sampling time we selected.
• 64MHz is the ADC clock.
• 64 is the OSR we selected.

The total conversion time for channel 8 should be 60uS. We will measure it in the analyzer.

ADC DMA Configuration

Below is the image showing the ADC DMA Configuration.

The DMA is configured in the Circular mode, so that it can work continuously. The data width is set to 16-bit.

Wiring Diagram for connecting the Potentiometer

Below is the image showing the connection used in this project.

The potentiometer is connected to 3.3V with the MCU. The output pin of the potentiometer is connected to the pin PA3 (ADC1 CH8). I am going to use the pin PD7 toggle to measure the ADC conversion time, therefore it is connected to the Logic Analyzer.

STM32 HAL Code for ADC Oversampling

There is nothing special in the programming section to make the oversampling work. We will simply start the ADC in the DMA mode.

uint16_t ADC_VAL = 0;

void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef *hadc)
{
    HAL_GPIO_TogglePin(GPIOD, GPIO_PIN_7);
}

int main(void)
{
  ....
  ....
  
  HAL_ADC_Start_DMA(&hadc1, &ADC_VAL, 1);
  while (1)
  {
  }
}

uint16_t ADC_VAL = 0;

void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef *hadc)
{
    HAL_GPIO_TogglePin(GPIOD, GPIO_PIN_7);
}

int main(void)
{
  ....
  ....
  
  HAL_ADC_Start_DMA(&hadc1, &ADC_VAL, 1);
  while (1)
  {
  }
}

Here inside the main function, we will start the ADC in the DMA mode. The converted data will be stored in the ADC_VAL variable.

Once the conversion is finished, an interrupt will trigger and the conversion complete callback is called. Inside the callback we will toggle the pin PD7. This is to measure the conversion time of the ADC.

Result of STM32 ADC Oversampling

Below is the gif showing the ADC_VAL in the live expression.

You can see in the gif above, as the potentiometer is rotated, the ADC_VAL is increasing. The interesting thing is that the maximum value of this variable is around 65000.

The STM32L496 supports a maximum of 12-bit ADC resolution. With 12-bit resolution, the maximum value of the variable should have been 4095. But because of oversampling, we are able to increase the resolution to 16-bit and that is why the value is reaching 65000.

Below is the image showing the sampling time as measured by the pin PD7.

As you can see above, the pin toggles every 60us. This is exactly the same time as we calculated using the formula.

The point is, Oversampling increases the number of samples taken per trigger by the factor OSR but it also reduces the overall sampling frequency by same factor. Hence we need to take in consideration the OSR factor while calculating the sampling or conversion frequency.

Video Tutorial

Conclusion

In this tutorial, we explored the ADC oversampling feature in STM32 microcontrollers and how it can be used to improve measurement quality without adding external hardware. We covered what oversampling is, how it increases effective resolution, the math behind resolution improvement, and how to configure the feature correctly using STM32CubeMX. Practical examples helped illustrate the trade-off between conversion speed and accuracy.

ADC oversampling is especially useful for slow-changing and noise-sensitive signals such as sensor readings, battery voltage monitoring, and precision measurements. When used with the right oversampling ratio and bit shift, it can significantly enhance ADC performance while keeping the design simple and cost-effective. Understanding its limitations and timing impact allows you to decide when oversampling is the right tool for your application.

STM32 ADC Part 3 – Multiple Channels with DMA Normal Mode

STM32 ADC Part 4 – Multiple Channels with DMA Circular Mode

STM32 ADC Part 5 – Read Multiple‑Channel without DMA

STM32 ADC Part 6 – ADC Conversion Time Explained

STM32 ADC PART 7 – ADC External Trigger Source Selection

STM32 ADC Part 8 – Injected Conversion Mode

STM32 ADC Part 10 – How to use ADC Reference Voltage

STM32 ADC Part 11 – How to use ADC in Differential Mode