AN3137 Datasheet, PDF (27/42 Pages) STMicroelectronics – Analog-to-digital converter on STM8L and STM8AL devices

AN3137
Methods for precision improvement
Figure 13. Noise observed on ADC input pin during ADC conversions
4.4.2
ADC input signal during
conversion: an ADC noise is
injected to the input.
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Explanation of the behavior
The explanation of this added pin noise and added measurement error (in case a signal
source with high internal impedance is used) comes from the internal ADC structure: its
input sampling circuit.
Figure 14 shows a simplified schematic of the input stage (sample and hold circuit).
Figure 14. ADC simplified schematic of input stage - sample and hold circuit
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#SH
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The spikes (noise) present on ADC input pin during conversions are related to the sampling
switch (S1). If the switch is closed, some charge (coming from the sample and hold
capacitor C_sh or caused by another effect) is transferred to the input pin. Then this charge
starts discharging through the source impedance (R_in). The discharge process ends at the
end of the sampling time (t_S) when the switch S1 is opened. The remaining undischarged
voltage remains on the capacitor C_sh and ADC measures this voltage. If the sampling time
(t_S) is too short, the remaining voltage does not drop under 0.5 lsb and ADC measurement
shows an added error. Figure 15 shows this process.
Doc ID 16983 Rev 3
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