External Reference Voltage Parameters; Analog Input Signal Noise; Adc Dynamic Range Bad Match For Maximum Input Signal Amplitude; Effect Of The Analog Signal Source Resistance - ST STM32 Application Note

ADC errors
2.2.3

External reference voltage parameters

In case of usage external source for reference voltage (on V
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
time. This noise is generated by electrical devices, such as motors, engine ignition, power
lines. It affects the source signal (such as sensors) by adding an unwanted signal. As a
consequence, the ADC conversion results are not accurate.
2.2.5

ADC dynamic range bad match for maximum input signal amplitude

To obtain the maximum ADC conversion precision, it is very important that the ADC dynamic
range matches the maximum amplitude of the signal to be converted. Let us assume that
the signal to be converted varies between 0 V and 2.5 V and that V
The maximum signal value converted by the ADC is 3102 (2.5 V) as shown in
this case, there are 993 unused transitions (4095 – 3102 = 993). This implies a loss in the
converted signal accuracy.
See Section 3.2.5: Matching the ADC dynamic range to the maximum signal amplitude on
page 26
signal amplitude.
2.2.6

Effect of the analog signal source resistance

The impedance of the analog signal source, or series resistance (R
and pin, causes a voltage drop across it because of the current flowing into the pin. The
charging of the internal sampling capacitor (C
resistance R
With the addition of source resistance (with R
capacitor increases.
18/49
for details on how to make the ADC dynamic range match the maximum input

Figure 14. Input signal amplitude vs. ADC dynamic range

.
ADC
Figure 15
) is controlled by switches with a
ADC
), the time required to fully charge the hold
ADC
shows the analog signal source resistance effect.
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pin) there are important
REF+
is equal to 3.3 V.
REF+
Figure
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