AN3029 datasheet(9/42 Pages) STMICROELECTRONICS

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AN3029

Analog-to-digital converter (ADC)

This section does not apply to STM8L101xx devices.

3.1

Analog power

For some packages, the ADC unit has an independent, analog supply voltage, isolated on

input pin VDDA, which allows the ADC to accept a very clean voltage source. This analog

voltage, VDDA, should be identical to the digital voltage supply on pin VDD. To filter some

noise, a ferrite bead can be added between VDD and VDDA. This ferrite bead should be

chosen according to the frequencies to be filtered.

Some packages also provide a separate external analog reference voltage input for the

ADC unit on the VREF+ pin. This gives better accuracy on low voltage input as follows:

•

The VREF+ pin can be connected to the VDDA external power supply. If a separate,

external reference voltage is applied on VREF+, a 100 nF and a 1 µF capacitor must be

connected on this pin. To compensate for peak consumption on VREF, the 1 µF

capacitor may be increased to 10 µF when the sampling speed is low. In all cases,

VREF+ must be kept between 2.4 V and VDDA. If VDDA is below 2.4, VREF+ must be

equal to VDDA. This input is internally bonded to VDDA in the devices that have no

internal VREF+ pin.

•

VREF- (input, analog reference negative): The lower/negative reference voltage is

internally bonded to VSSA.

3.2

Analog input

The devices include up to 28 analog input channels (including four fast channels), each

multiplexed with an I/O, which are converted by the ADC one at a time.

The external input impedance (RAIN) max value is 50 kΩ. The four fast channels can convert

at maximum speed (1 MHz) if RAIN is less than 0.5 kΩ.

Please refer to Figure 3.

Figure 3. Analog input interface

Please refer to the applicable datasheets and reference manuals for more details.

MS32530V1

Sample and hold

ADC converter

12-bit

converter

RADC

CADC

0.6 V

VDD

AINx

RAIN

Cparasitic

VAIN

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