AN3137 datasheet(11/42 Pages) STMICROELECTRONICS

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AN3137

STM8L/STM8AL ADC internal hardware

Doc ID 16983 Rev 3

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1.4

Power supply range

The ADC is powered from VDDA/VSSA pins (usually connected to VDD/VSS - depending on

packages). The voltage range for a proper ADC operation is from 1.8 V to 3.6 V (the digital

part operates from 1.65 V). See the electrical characteristics in the STM8L/STM8AL device

datasheets for further details.

1.5

ADC reference voltage

The ADC reference voltage is either internally connected to the analog power supply pins or

connected to the VREF+ external pin, where an external reference voltage source can be

connected. This reference voltage connection option depends on the STM8L/STM8AL

package and device types:

●

In case VDDA supply voltage is greater than 2.4 V, the VREF+ pin can be connected to a

voltage different from VDDA. VREF+ must then be greater or equal to 2.4 V and less or

equal to VDDA (VDDA ≥VREF+ ≥ 2.4 V).

●

In case VDDA supply voltage is less than 2.4 V, VREF+ must have the same voltage as

VDDA.

As the ADC reference voltage highly influences ADC precision, these voltage properties are

an important characteristic to be taken into account in the application design (stability,

noise,...).

The ADC can convert the VREFINT internal reference voltage (on ADC channel 25) to

perform calibration (especially when VDD/VSS are used as reference voltage). See

Section 2.3.2: Correction to VDD/VREF stability for more information.

1.6

Input analog multiplexer

The ADC has an input multiplexer which is used to select one of the STM8L/STM8AL input

pins as the analog input to the ADC. 25 external channels (on I/O pins) and 2 internal

channels (internal reference voltage and temperature sensor) are available.

The Schmitt triggers can be disabled on selected ADC input pins (mainly used in digital I/O

pin configuration). The STM8L/STM8AL power consumption can then decrease but the

digital inputs on the corresponding I/O pins will not work.

1.6.1

Internal channels

Internal analog channels (internal reference voltage and temperature sensor) can be used

for calibration purposes (or for temperature measurement). These analog sources must be

enabled before using them (they are disabled by default so as to save power).

1.6.2

Fast/slow channels

Analog channels are divided into two groups: a fast channel sequence and a slow channel

sequence.

Each channel sequence has an own individual setting for sampling time (two different

sampling times can be set: one for each channel sequence).This feature is useful when

some measured voltage sources require a longer sampling time (e.g. source with a high

internal impedance). Refer to Section 2.3.3: Measurement of sources with high internal

impedance).

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