ADC08231 datasheet(13/24 Pages) NSC

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Functional Description

10 MULTIPLEXER ADDRESSING

The design of these converters utilizes a comparator struc-

ture with built-in sample-and-hold which provides for a dif-

ferential analog input to be converted by a successive-

approximation routine

The actual voltage converted is always the difference be-

tween an assigned ‘‘a’’ input terminal and a ‘‘b’’ input ter-

minal The polarity of each input terminal of the pair indi-

cates which line the converter expects to be the most posi-

tive If the assigned ‘‘a’’ input voltage is less than the ‘‘b’’

input voltage the converter responds with an all zeros out-

put code

A unique input multiplexing scheme has been utilized to pro-

vide multiple analog channels with software-configurable

single-ended differential or pseudo-differential (which will

convert the difference between the voltage at any analog

input and a common terminal) operation The analog signal

conditioning required in transducer-based data acquisition

systems is significantly simplified with this type of input flexi-

bility One converter package can now handle ground refer-

enced inputs and true differential inputs as well as signals

with some arbitrary reference voltage

A particular input configuration is assigned during the MUX

addressing sequence prior to the start of a conversion The

MUX address selects which of the analog inputs are to be

enabled and whether this input is single-ended or differen-

tial Differential inputs are restricted to adjacent channel

pairs For example channel 0 and channel 1 may be select-

ed as a differential pair but channel 0 or 1 cannot act

differentially with any other channel In addition to selecting

differential mode the polarity may also be selected Channel

0 may be selected as the positive input and channel 1 as

the negative input or vice versa This programmability is

best illustrated by the MUX addressing codes shown in the

following tables for the various product options

The MUX address is shifted into the converter via the DI

line Because the ADC08231 contains only one differential

input channel with a fixed polarity assignment it does not

require addressing

The common input line (COM) on the ADC08238 can be

used as a pseudo-differential input In this mode the voltage

on this pin is treated as the ‘‘b’’ input for any of the other

input channels This voltage does not have to be analog

ground it can be any reference potential which is common

to all of the inputs This feature is most useful in single-sup-

ply applications where the analog circuitry may be biased up

to a potential other than ground and the output signals are

all referred to this potential

TABLE I MultiplexerPackage Options

Part

Number of Analog Channels

Number of

Number

Single-Ended

Differential

Package Pins

ADC08231

ADC08234

ADC08238

TABLE II MUX Addressing ADC08238

Single-Ended MUX Mode

MUX Address

Analog Single-Ended Channel

START

SGL

ODD

SELECT

01234567

COM

DIF

SIGN

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