ST7262 datasheet(121/132 Pages) STMICROELECTRONICS

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ST7262

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ADC CHARACTERISTICS (Cont’d)

12.12.0.1 Analog Power Supply and Reference

Pins

Depending on the MCU pin count, the package

may feature separate VDDA and VSSA analog pow-

er supply pins. These pins supply power to the A/D

converter cell and function as the high and low ref-

erence voltages for the conversion. In smaller

packages VDDA and VSSA pins are not available

and the analog supply and reference pads are in-

ternally bonded to the VDD and VSS pins.

Separation of the digital and analog power pins al-

low 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

10.7.3.2 PCB Design Guidelines).

12.12.0.2 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 the noise-sensi-

tive, analog physical interface from noise-generat-

ing CMOS logic signals.

– Use separate digital and analog planes. The an-

alog ground plane should be connected to the

digital ground plane via a single point on the

PCB.

– Filter power to the analog power planes. The

best solution is to connect capacitors, with good

high frequency characteristics, between the

power and ground lines, placing 0.1µF and 10pF

capacitors as close as possible to the ST7 power

supply pins and a 10µF capacitor close to the

power source (see Figure 89).

– The analog and digital power supplies should be

connected in a star nework. Do not use a resis-

tor, as VDDA 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 out-

puts on the same I/O port as the A/D input being

converted.

12.12.0.3

Specific

Application

Design

Guidelines

– When a USB transmission is taking place during

A/D conversion, the noise caused on the analog

power supply by the USB transmission may re-

sult in a loss of ADC accuracy.

– If the USB is used to supply power to the appli-

cation, this causes noise which may result in a

loss of ADC accuracy.

Figure 89. Power Supply Filtering

VSS

VDD

0.1

µF

10pF

VDD

ST72XXX

VDDA

VSSA

POWER

SUPPLY

SOURCE

ST7

DIGITAL NOISE

FILTERING

EXTERNAL

NOISE

FILTERING

µF

0.1

µF

10pF

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