AN2834 Datasheet, PDF(17/45 Page) STMicroelectronics – How to get the best ADC accuracy in STM32Fx Series and STM32L1 Series devices

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AN2834 Datasheet, PDF (17/45 Pages) STMicroelectronics – How to get the best ADC accuracy in STM32Fx Series and STM32L1 Series devices

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AN2834

Figure 15. Analog signal source resistance effect

STM32

VAIN

RAIN AINx

RADC

12-bit ADC

CADC

VSSA

ADC errors

VAIN

2.2.7

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1. tVtccc=i:sc(tRahpeAaDtciCmit+oerRt(aCAkIAeNDn)CÃb)yCvotAhlDteaCgCeAï DC capacitor to fully charge: Vc = VAIN (with max.1/2 lsb error)ï

Effect of source capacitance and parasitic capacitance of the PCB

When converting analog signals, it is necessary to account for the capacitance at the source

and the parasitic capacitance seen on the analog input pin (refer to Figure 16). The source

resistance and capacitance form an RC network and the ADC conversion results may not be

accurate unless the external capacitor (CAIN + Cp) is fully charged to the level of the input

voltage. The greater value of (CAIN + Cp), the more limited the source frequency.

The external capacitance at the source and the parasitic capacitance are denoted by CAIN

and Cp, respectively.

Figure 16. Analog input with RAIN, CAIN and Cp

VAIN

Source

RAIN

CAIN

STM32

AINx

ai15483

DocID15067 Rev 2

17/45

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