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AN2834 How to get the best ADC accuracy
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3.4 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.
3.4.1 ADC input stage problem
The ADC embedded in STM32 devices is a switched-capacitor ADC. 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 kΩ), an additional error can be seen in measurement results. Error signals have also
b
een observed on the ADC input pin, as shown in Figure 33 (if the voltage source has zero
voltage: U
in
= 0 V, R
in
= 150 kΩ, C
ext
= 0 pF):
Figure 31. Typical voltage source connection to ADC input
Figure 32. Noise observed on ADC input pin during ADC conversions
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