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ST STM32 Application Note

How to get the best adc accuracy in microcontrollers
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Introduction
STM32 microcontrollers embed up to four advanced 12-bit ADCs (depending on the
device). A self-calibration feature is provided to enhance ADC accuracy versus
environmental condition changes.
In applications involving analog-to-digital conversion, ADC accuracy has an impact on the
overall system quality and efficiency. To improve this accuracy, the errors associated with
the ADC and the parameters affecting them must be understood.
ADC accuracy does not only depend on ADC performance and features, but also on the
overall application design around the ADC.
This application note aim is to help understand ADC errors and explain how to enhance
ADC accuracy. It is divided into three main parts:
• a simplified description of ADC internal structure to help understand ADC operation and
related ADC parameters
• explanations of the different types and sources of ADC errors related to the ADC design
and to external ADC parameters such as the external hardware design
• recommendations on how to minimize these errors, focusing on hardware and software
methods
February 2017
How to get the best ADC accuracy
in STM32 microcontrollers
DocID15067 Rev 3
AN2834
Application note
1/49
www.st.com
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  • Page 1 How to get the best ADC accuracy in STM32 microcontrollers Introduction STM32 microcontrollers embed up to four advanced 12-bit ADCs (depending on the device). A self-calibration feature is provided to enhance ADC accuracy versus environmental condition changes. In applications involving analog-to-digital conversion, ADC accuracy has an impact on the overall system quality and efficiency.

  • Page 2: Table Of Contents

    Contents AN2834 Contents ADC internal principle ........6 SAR ADC internal structure .
  • Page 3 AN2834 Contents 3.2.9 Minimizing injection current ........31 3.2.10 Minimizing I/O pin crosstalk .
  • Page 4 List of tables AN2834 List of tables Table 1. Document revision history ..........48 4/49 DocID15067 Rev 3...
  • Page 5 AN2834 List of figures List of figures Figure 1. Basic schematic of SAR switched-capacitor ADC (example of 10-bit ADC)... . . 6 Figure 2. Sample state ............. 7 Figure 3.

  • Page 6: Adc Internal Principle

    ADC internal principle SAR ADC internal structure The ADC embedded in STM32 microcontrollers uses the SAR (successive approximation register) principle, by which the conversion is performed in several steps. The number of conversion steps is equal to the number of bits in the ADC converter. Each step is driven by the ADC clock.

  • Page 7: Figure 2. Sample State

    AN2834 ADC internal principle Figure 2. Sample state 1. Sample state: capacitors are charging to V voltage. Sa switched to V , Sb switch closed during sampling time. Figure 3. Hold state 1. Hold state: the input is disconnected, capacitors hold input voltage. Sb switch is open, then S1-S11 switched to ground and Sa switched to V DocID15067 Rev 3 7/49...

  • Page 8: Figure 4. Step 1: Compare With Vref/2

    ADC internal principle AN2834 Figure 4. Step 1: Compare with V 1. First approximation step. S1 switched to V Figure 5. Step 2: If MSB = 0, then compare with ¼V 1. Compare with ¼V ; if MSB =1. S1 switched back to ground. S2 switched to V 8/49 DocID15067 Rev 3...

  • Page 9: Figure 6. Step 2: If Msb = 1, Then Compare With ¾Vref

    AN2834 ADC internal principle Figure 6. Step 2: If MSB = 1, then compare with ¾V 1. Compare with ¾V ; if MSB =0. S1 remained switched to ground. S2 switched to V DocID15067 Rev 3 9/49...

  • Page 10: Adc Errors

    1. Still ideally, the first transition occurs at 0.5 LSB. The offset error is denoted by . The offset error can easily be calibrated by the application firmware. Example For the STM32 ADC, the smallest detectable incremental change in voltage is expressed in terms of LSBs: 1 LSB = V...

  • Page 11: Figure 7. Positive Offset Error Representation

    AN2834 ADC errors Figure 7. Positive offset error representation 1. The error offset, E , is shown in magenta. When an analog input voltage of less than 0.5 LSB generates the first transition, the offset error is negative (refer to Figure 8 for an example of negative offset error).

  • Page 12: Gain Error

    ADC errors AN2834 2.1.2 Gain error The gain error is the deviation between the last actual transition and the last ideal transition. It is denoted by E The last actual transition is the transition from 0xFFE to 0xFFF. Ideally, there should be a transition from 0xFFE to 0xFFF when the analog input is equal to V –...

  • Page 13: Differential Linearity Error

    AN2834 ADC errors Figure 10. Negative gain error representation 1. The gain error, E , is shown in magenta. 2.1.3 Differential linearity error The differential linearity error (DLE) is the maximum deviation between the actual and ideal steps. Here ‘ideal’ does not refer to the ideal transfer curve but to the ADC resolution. The DLE is denoted by E .

  • Page 14: Integral Linearity Error

    ADC errors AN2834 Figure 11. Differential linearity error representation 1. The differential linearity error, E , is shown in magenta. If V = 3.3 V, an analog input of 1.9998 V (0x9B1) can provide results varying between REF+ 0x9B0 and 0x9B2. Similarly, for an input of 2.0014 V (0x9B3), the results may vary between 0x9B2 and 0x9B4.

  • Page 15: Figure 12. Integral Linearity Error Representation

    AN2834 ADC errors Figure 12. Integral linearity error representation 1. The integral linearity error, E , is shown in magenta. Example If the first transition from 0 to 1 occurs at 550 µV and the last transition (0xFFE to 0xFFF) occurs at 3.298435 V (gain error), then the line on the transfer curve that connects the actual digital codes 0x1 and 0xFFF is the endpoint correlation line.

  • Page 16: Total Unadjusted Error

    ADC errors AN2834 2.1.5 Total unadjusted error The total unadjusted error (TUE) is the maximum deviation between the actual and the ideal transfer curves. This parameter specifies the total errors that may occur, thus causing the maximum deviation between the ideal digital output and the actual digital output. TUE is the maximum deviation recorded between the ideal expected value and the actual value obtained from the ADC for any input voltage.

  • Page 17: Errors Due To The Adc Environment

    AN2834 ADC errors Errors due to the ADC environment 2.2.1 Reference voltage noise As the ADC output is the ratio between the analog signal voltage and the reference voltage, any noise on the analog reference causes a change in the converted digital value. V analog power supply is used on some packages as the reference voltage (V ), so the REF+...

  • Page 18: External Reference Voltage Parameters

    ADC errors AN2834 2.2.3 External reference voltage parameters In case of usage external source for reference voltage (on V pin) there are important REF+ parameters of this external reference source. Three reference voltage specifications must be considered: temperature drift, voltage noise, long term stability. 2.2.4 Analog input signal noise Small but high-frequency signal variation can result in big conversion errors during sampling...

  • Page 19: Effect Of Source Capacitance And Parasitic Capacitance Of The Pcb

    AN2834 ADC errors The effective charging of C is governed by R , so the charging time constant becomes t = (R ) × C . If the sampling time is less than the time required to fully charge the C through R <...

  • Page 20: Injection Current Effect

    ADC errors AN2834 Figure 16. Analog input with R and C 2.2.8 Injection current effect A negative injection current on any analog pin (or a closely positioned digital input pin) may introduce leakage current into the ADC input. The worst case is the adjacent analog channel.

  • Page 21: I/O Pin Crosstalk

    AN2834 ADC errors 2.2.10 I/O pin crosstalk Switching the I/Os may induce some noise in the analog input of the ADC due to capacitive coupling between I/Os. Crosstalk may be introduced by PCB tracks that run close to each other or that cross each other. Internally switching digital signals and I/Os introduces high-frequency noise.

  • Page 22: How To Get The Best Adc Accuracy

    ADC curve stored in the microcontroller memory but this needs calibration of each individual device in final application. Offset and gain errors can be easily compensated using the STM32 ADC self-calibration feature or by microcontroller firmware.

  • Page 23: Figure 20. Power Supply And Reference Decoupling For 100- And 144-Pin Packages

    1 µF Tantalum or Ceramic). Refer to Figure 20 Figure 21 for decoupling examples. For STM32 microcontrollers delivered in 100/144-pin packages, it is possible to improve the accuracy on low-voltage inputs by connecting a separate external ADC reference voltage input on V (refer to Section 3.2.5).

  • Page 24: Reference Voltage / Power-Supply Regulation

    How to get the best ADC accuracy AN2834 3.2.2 Reference voltage / Power-supply regulation The power supply should have good line and load regulation since the ADC uses V REF+ as the analog reference and the digital value is the ratio of the analog input signal to this voltage reference.

  • Page 25: Adding White Noise Or Triangular Sweep To Improve Resolution

    (so, capacitive coupling must be used). A very simple implementation of the quasi-triangular source which is generated directly by the STM32 microcontroller is on Figure Figure 22. Simple quasi-triangular source using a microcontroller output...

  • Page 26: Matching The Adc Dynamic Range To The Maximum Signal Amplitude

    If the measured signal is too small (in comparison with the ADC range), then an external preamplifier can be useful. This method can be implemented whatever the STM32 package, and more specifically in packages that do not have a V input.

  • Page 27: Figure 24. Preamplification

    AN2834 How to get the best ADC accuracy This amplifier can adapt the input signal range to the ADC range. It can also insert offsets between the input signal and the ADC input. When designing the preamplifier, care must be taken not to generate additional errors (such as additional offset, amplifier gain stability or linearity, frequency response).

  • Page 28: Analog Source Resistance Calculation

    How to get the best ADC accuracy AN2834 3.2.6 Analog source resistance calculation Let us assume that the maximum error allowed is equal to 1/2 LSB. We will calculate the maximum source resistance allowed. is the voltage across the internal C capacitor (refer to Figure 15).
  • Page 29 AN2834 How to get the best ADC accuracy By combining equations (1), (2) and (3), we obtain final formula: ------------------------------------------------------------------------ - R – AINmax ADCmax ⋅ ⋅ Example for STM32F1 parameters = 14 MHz, C = 8 pF, R max = 1 kΩ and for T = 7.5, the maximum source resistance allowed for an error equal to 1/2 LSB is: ----------------------------------------------------------------------------------------------- - 1kΩ...

  • Page 30: Source Frequency Condition Vs. Source And Parasitic Capacitors

    How to get the best ADC accuracy AN2834 3.2.7 Source frequency condition vs. source and parasitic capacitors The external capacitance will not allow the analog input voltage to be exactly the same as if the capacitor is not fully charged by the analog source. If the analog input signal changes, then the analog signal frequency (F ) should be such that the time period of this analog signal is at least: 10 ×...

  • Page 31: Temperature-Effect Compensation

    I/O pin that can give birth to the negative injection current. The ADC accuracy will not be affected by positive injection currents within the limits specified for I and ΣI (refer to the appropriate STM32 datasheet, I/O port INJ(PIN) INJ(PIN) characteristics section). 3.2.10...

  • Page 32: Emi-Induced Noise Reduction

    How to get the best ADC accuracy AN2834 3.2.11 EMI-induced noise reduction You can reduce EMI noise using proper shielding and layout techniques. The possible sources of emission must be physically separated from the receptors. They can be separated electrically by proper grounding and shielding. Shielding technique Placing ground tracks alongside sensitive analog signals provides shielding on the PCB.

  • Page 33: Pcb Layout Recommendations

    (see Figure 30). Depending on the STM32 package, different analog and digital power supply and ground pins are available. The V and V pins can be powered from separate power supplies.

  • Page 34: Figure 30. Separating The Analog And Digital Supplies

    How to get the best ADC accuracy AN2834 Separating the analog and digital supplies Figure 30. It is also recommended to connect the analog and digital grounds in a star network. This means that you must connect the analog and digital grounds at only one point. This prevents the introduction of noise in the analog power supply circuit due to digital signal switching.

  • Page 35: Component Placement And Routing

    AN2834 How to get the best ADC accuracy 3.2.13 Component placement and routing Place the components and route the signal traces on the PCB so as to shield analog inputs. Components like resistors and capacitors must be connected with very short leads. You can use surface-mounted device (SMD) resistors and capacitors.

  • Page 36: Digital Signal Filtering

    How to get the best ADC accuracy AN2834 In STM32L0 and STM32L4 microcontrollers, averaging can be performed by using the hardware oversampling feature: the ADC performs built-in hardware averaging according to configurable parameters (number of samples to average and final right bit shift of result). The advantage of averaging is to improve ADC precision without any hardware changes.

  • Page 37: Fft For Ac Measurement

    AN2834 How to get the best ADC accuracy 3.3.3 FFT for AC measurement In some specific cases the application needs to know the amplitude of an AC signal with a given frequency. In this case the effective value of an AC signal can also be obtained by using a relatively slow sampling speed (in comparison to the measured signal frequency).

  • Page 38: Adc Calibration

    How to get the best ADC accuracy AN2834 3.3.4 ADC calibration This method requires knowledge of the internal ADC structure and of how the ADC converter is implemented inside the microcontroller. This is necessary to design a physical/mathematical model of the ADC implementation. A proper physical model (which is usually a schematic diagram) is used as a basis for describing it mathematically.

  • Page 39: High Impedance Source Measurement

    How to get the best ADC accuracy High impedance source measurement This section describes the ADC measurement behavior of STM32 ADC when a signal source with high internal impedance is used. It explains how to design an application to reach the requested precision and provides workarounds.

  • Page 40: Explanation Of The Behavior

    How to get the best ADC accuracy AN2834 3.4.2 Explanation of the behavior The explanation of this additional pin noise and additional measurement error (in case a signal source with high internal impedance is used) comes from the internal ADC structure: its input sampling circuit.

  • Page 41: Minimizing Additional Errors

    AN2834 How to get the best ADC accuracy 3.4.3 Minimizing additional errors Workaround for high impedance sources To solve the additional error problem, the sampling time (T ) can be increased by configuring ADC settings in MCU firmware, so that the C charge is discharged through the source impedance R .

  • Page 42: Figure 36. Charging The External Capacitor With Too Short Time Between Conversions

    How to get the best ADC accuracy AN2834 Workaround for extra high impedance sources This workaround combines both hardware and software changes. Hardware change The hardware change consists in adding a large external capacitor (C ) to the input pin. The capacity size connected to the input pin must reach the value that causes the discharging of the internal sampling capacity C to the external capacitor C...
  • Page 43 AN2834 How to get the best ADC accuracy Software change The side effect mentioned above can be solved by software. The objective is to create a delay in order to let C discharge through R (not measure so often) giving enough “discharge time”...
  • Page 44 How to get the best ADC accuracy AN2834 An extra large C >> ) enables sampling more often. ⋅ ------------- - However, increasing C limits the frequency bandwidth of measurement signal (increasing the “external” timing constant R The formulas below show how to choose the optimal C value: signal bandwidth in correlation with sample time.

  • Page 45: Source Of Described Problem - Adc Design

    AN2834 How to get the best ADC accuracy = 4096 LSB ..ADC property = 0.5 LSB ..required precision 4096 ⋅ ⋅ ⋅ ≈ ⇒ 1 58 1 58 16pF 207nF 220nF ------------- - ------------ - 16pF 4096 ⋅ ⋅...

  • Page 46: Figure 38. Parasitic Capacitances Of Sampling Switch

    How to get the best ADC accuracy AN2834 Figure 38. Parasitic capacitances of sampling switch This charging and discharging currents (PMOS and NMOS asymmetric capacitances) can cause charge transfer to sampling capacitor C Internal charging of sampling capacitor It is possible that after the conversion process (successive approximation process in SAR type of ADC) the sample and hold capacitor C is charged to some voltage.

  • Page 47: Conclusion

    Conclusion This application notes describes the main ADC errors and then methods and application design rules to minimize STM32 ADC errors and obtain the best ADC accuracy. The choice of method depends on the application requirements and is always a compromise between speed, precision, enough computation power and design topology.

  • Page 48: Revision History

    Text improvements and additions. Changed the Disclaimer on the final page. Document scope extended to all STM32 microcontrollers. Updated Figure 5: Step 2: If MSB = 0, then compare with ¼VREF Figure 6: Step 2: If MSB = 1, then compare with ¾VREF.
  • Page 49 ST products and/or to this document at any time without notice. Purchasers should obtain the latest relevant information on ST products before placing orders. ST products are sold pursuant to ST’s terms and conditions of sale in place at the time of order acknowledgement.

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