ST7262 datasheet(88/132 Pages) STMICROELECTRONICS

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ST7262

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10.7 10-BIT A/D CONVERTER (ADC)

10.7.1 Introduction

The on-chip Analog to Digital Converter (ADC) pe-

ripheral is a 10-bit, successive approximation con-

verter with internal sample and hold circuitry. This

peripheral has up to 8 multiplexed analog input

channels (refer to device pin out description) that

allow the peripheral to convert the analog voltage

levels from up to 8 different sources.

The result of the conversion is stored in a 10-bit

Data Register. The A/D converter is controlled

through a Control/Status Register.

10.7.2 Main Features

10-bit conversion

Up to 8 channels with multiplexed input

Linear successive approximation

Data register (DR) which contains the results

Conversion complete status flag

Continuous or One-Shot mode

On/off bit (to reduce consumption)

The block diagram is shown in Figure 51.

10.7.3 Functional Description

10.7.3.1 Analog Power Supply

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.

10.7.3.2 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. The analog power plane should be con-

nected to the digital power plane via an RC net-

work.

– Filter power to the analog power planes. The

best solution is to connect a 0.1µF capacitor, with

good high frequency characteristics, between

VDDA and VSSA and place it as close as possible

to the VDDA and VSSA pins and connect the ana-

log and digital power supplies in a star network.

Do not use a resistor, as VDDA is used as a refer-

ence voltage by the A/D converter and resist-

ance 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 signal 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 convert-

ed.

10.7.3.3 Digital A/D Conversion Result

The conversion is monotonic, meaning that the re-

sult never decreases if the analog input does not

and never increases if the analog input does not.

If the input voltage (VAIN) is greater than VDDA

(high-level voltage reference) then the conversion

result is FFh in the ADCDRMSB register and 03h

in the ADCDRLSB register (without overflow indi-

cation).

If the input voltage (VAIN) is lower than VSSA (low-

level voltage reference) then the conversion result

in the ADCDRMSB and ADCDRLSB registers is

00 00h.

The A/D converter is linear and the digital result of

the conversion is stored in the ADCDRMSB and

ADCDRLSB registers. The accuracy of the con-

version is described in the Electrical Characteris-

tics Section.

RAIN is the maximum recommended impedance

for an analog input signal. If the impedance is too

high, this will result in a loss of accuracy due to

leakage and sampling not being completed in the

alloted time.

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