AN2834 datasheet(35/45 Pages) STMICROELECTRONICS

Part #	AN2834

Description	How to get the best ADC accuracy in STM32Fx Series and STM32L1 Series devices

PDF	45 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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AN2834

How to get the best ADC accuracy

3.4

High impedance source measurement

This section describes the ADC measurement behavior of STM32x 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.

3.4.1

ADC input stage problem

The ADC in STM32x devices is a switched-capacitor ADC type. Switched capacitors work

also as sampling capacitors (see Section 1.1 for a detailed explanation).

When a signal comes from a voltage source with high internal impedance (for instance,

150 k

), an added error can be seen in measurement results. Error signals have also been

observed on the ADC input pin, as shown in Figure 33 (if the voltage source has zero

voltage: Uin = 0 V, Rin = 150 k, Cext = 0 pF):

Figure 31. Typical voltage source connection to ADC input

Figure 32. Noise observed on ADC input pin during ADC conversions

ADC input

ai17903

STM32

Uin

Rin

C ext

ADC input signal during conversion:

an ADC noise is injected to the input.

ai17904

AN2834 Datasheet(PDF) 35 Page - STMicroelectronics