ADC08138CIWM datasheet(12/21 Pages) NSC

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Functional Description

MULTIPLEXER ADDRESSING

The design of these converters utilizes a comparator struc-

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

ential analog input to be converted by a successiveapproxi-

mation routine.

The actual voltage converted is always the difference be-

tween an assigned “+” input terminal and a “−” input terminal.

The polarity of each input terminal of the pair indicates which

line the converter expects to be the most positive. If the as-

signed “+” input voltage is less than the “−” input voltage the

converter responds with an all zeros output 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 in-

put and a common terminal) operation. The analog signal

conditioning required in transducer-based data acquisition

systems is significantly simplified with this type of input flex-

ibility. 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 differential.

Differential inputs are restricted to adjacent channel pairs.

For example, channel 0 and channel 1 may be selected 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 ADC08131 contains only one differential input

channel with a fixed polarity assignment, it does not require

addressing.

The common input line (COM) on the ADC08138 can be

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

on this pin is treated as the “−” 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-supply applica-

tions where the analog circuity may be biased up to a poten-

tial other than ground and the output signals are all referred

to this potential.

TABLE 1. Multiplexer/Package Options

Part

Number of Analog Channels

Number of

Number

Single-Ended

Differential

Package

Pins

ADC08131

ADC08134

ADC08138

TABLE 2. MUX Addressing: ADC08138

Single-Ended MUX Mode

MUX Address

Analog Single-Ended Channel #

START

SGL/

ODD/

SELECT

01234567

COM

DIF

SIGN

−

TABLE 3. MUX Addressing: ADC08138

Differential MUX Mode

MUX Address

Analog Differential Channel-Pair #

START

SGL/

ODD/

SELECT

0123

DIF

SIGN

1001234567

−

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