AN3137 datasheet(28/42 Pages) STMICROELECTRONICS

Part #	AN3137

Description	Analog-to-digital converter on STM8L and STM8AL devices

PDF	42 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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Methods for precision improvement

AN3137

28/42

Doc ID 16983 Rev 3

Figure 15.

ADC input pin noise spikes from internal charge during sampling process

Note that a non-zero external capacitance Cext (parasitic pin capacitance) also exists, so

during conversion time the pin capacitance is discharged through source impedance Rin.

4.4.3

Minimizing added errors

Workaround for high impedance sources

To solve the added error problem, you can increase the sampling time (tS) configuring ADC

settings in MCU firmware, so that you discharge the Csh charge through the source

impedance Rin. The time constant (Rin * Csh) is the reference for choosing the sampling

time. To choose the sampling time cycles, use this formula (see also Section 1):

The clock for ADC (fADC) is another important factor; slowing down the ADC clock

increases the sampling time.

ADC

⋅

() In

4096

0.5

-------------

⋅⋅

≥

cycles

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AN3137 Datasheet(PDF) 28 Page - STMicroelectronics