ST72324BJ2 datasheet(165/188 Pages) STMICROELECTRONICS

165 / 188 page

ST72324B

Electrical characteristics

165/188

12.13.1

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. In some packages, VAREF and

VSSA pins are not available (refer to Section 2 on page 10). In this case the analog supply

and reference pads are internally bonded to the VDD and VSS pins.

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 12.13.2: General PCB

design guidelines).

12.13.2

General PCB design guidelines

To obtain best results, some general design and layout rules should be followed when

designing the application PCB to shield the noise-sensitive, analog physical interface from

noise-generating CMOS logic signals.

●

Use separate digital and analog planes. The analog ground plane should be connected

to the digital ground plane via a single point on the PCB.

●

Filter power to the analog power planes. It is recommended to connect capacitors, with

good high frequency characteristics, between the power and ground lines, placing

0.1µF and optionally, if needed 10pF capacitors as close as possible to the ST7 power

supply pins and a 1 to 10µF capacitor close to the power source (see Figure 87).

●

The analog and digital power supplies should be connected in a star network. Do not

use a resistor, as VAREF is used as a reference voltage by the A/D converter and any

resistance would cause a voltage drop and a loss of accuracy.

●

Properly place components and route the signal traces on the PCB to shield the analog

inputs. Analog signals paths should run over the analog ground plane and be as short

as possible. Isolate analog signals from digital signals that may switch while the analog

inputs are being sampled by the A/D converter. Do not toggle digital outputs on the

same I/O port as the A/D input being converted.

Figure 87.

Power supply filtering

VSS

VDD

ST72XXX

VAREF

VSSA

Power

supply

source

ST7

digital noise

filtering

External

noise

filtering

1 to 10µF

0.1µF

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