ST72321R7-Auto datasheet(217/243 Pages) STMICROELECTRONICS

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ST72321xx-Auto

Electrical characteristics

Doc ID 13829 Rev 1

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Figure 99.

Typical A/D converter application

19.12.1

Analog power supply and reference pins

Depending on the MCU pin count, the package may feature separate VAREF and VSSA

analog power supply pins. These pins supply power to the A/D converter cell and function

as the high and low reference voltages for the conversion.

Separation of the digital and analog power pins allow board designers to improve A/D

performance. Conversion accuracy can be impacted by voltage drops and noise in the event

of heavily loaded or badly decoupled power supply lines (see Section 19.12.2: General PCB

design guidelines).

Figure 97.

RAIN maximum versus fADC with

CAIN =0pF

(1)

Figure 98.

Recommended CAIN and RAIN

values(1)

CPARASITIC represents the capacitance of the PCB

(dependent on soldering and PCB layout quality) plus

the pad capacitance (3pF). A high CPARASITIC value will

downgrade conversion accuracy. To remedy this, fADC

should be reduced.

This graph shows that, depending on the input signal

variation (fAIN), CAIN can be increased for stabilization

time and decreased to allow the use of a larger serial

resistor (RAIN).

010

CPARASITIC (pF)

2 MHz

1 MHz

0.1

100

1000

0.01

0.1

fAIN(KHz)

Cain 10 nF

Cain 22 nF

Cain 47 nF

AINx

ST72XXX

VDD

Ilkg

0.6V

CADC

12pF

VAIN

RAIN

10-bit A/D

conversion

max

CAIN

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