AN3137 Datasheet, PDF (28/42 Pages) STMicroelectronics – Analog-to-digital converter on STM8L and STM8AL devices

Methods for precision improvement
AN3137
Figure 15. ADC input pin noise spikes from internal charge during sampling process
4.4.3
3AMPLINGTIMET₃

$ISCHARGINGTIME
#ONVERSION TIME
T_#
T
AI
Note that a non-zero external capacitance C_ext (parasitic pin capacitance) also exists, so
during conversion time the pin capacitance is discharged through source impedance R_in.
Minimizing added errors
Workaround for high impedance sources
To solve the added error problem, you can increase the sampling time (t_S) configuring ADC
settings in MCU firmware, so that you discharge the C_sh charge through the source
impedance R_in. The time constant (R_in * C_sh) is the reference for choosing the sampling
time. To choose the sampling time cycles, use this formula (see also Section 1):
t_S ≥ f_ADC ⋅
(R_in ⋅
C_sh) ⋅
In
4_----0_---9_---6_--
0.5
[cycles]
The clock for ADC (fADC) is another important factor; slowing down the ADC clock
increases the sampling time.
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Doc ID 16983 Rev 3