AN3137 datasheet(33/42 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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33 / 42 page

AN3137

Methods for precision improvement

Doc ID 16983 Rev 3

33/42

4.4.4

Source of described problem - ADC design

The following sections list some possible causes for the charging of the internal sampling

capacitor Csh. This is not an exhaustive list; only the main possible sources of the ADC

design are listed.

Parasitic switch capacitance effect

The sampling switch inside ADC sampling circuit (see Figure 14) is not ideal. In reality the

sample and hold switch (S1) is designed as 2 transistors (PMOS and NMOS):

Figure 18.

Implementation of sampling switch

The switch is controlled by the gate voltages of transistors (inverted signal on PMOS

transistor). This design is a standard bidirectional switch (for rail to rail range of input Uin

voltages).

Both transistors have parasitic capacitances between gate and source. If those

capacitances are charged (close to the switch), then their charge can be transferred to the

sampling capacitor.

AN3137 Datasheet(PDF) 33 Page - STMicroelectronics