ML2284BCS View Datasheet(PDF) -

ML2281, ML2282, ML2284, ML2288

FUNCTIONAL DESCRIPTION

MULTIPLEXER ADDRESSING

The design of these converters utilizes a sample data

comparator structure which provides for a differential

analog input to be converted by a successive

approximation routine.

The actual voltage converted is always the difference

between an assigned “+” input terminal and a “–” input

terminal. The polarity of each input terminal of the pair

being converted indicates which line the converter expects

to be the most positive. If the assigned “+” input is less than

the “–” input, the converter responds with an all zeros

output code.

A unique input multiplexing scheme has been utilized

to provide multiple analog channels with software

configurable single ended, differential, or pseudo

differential options. The pseudo differential option will

convert the difference between the voltage at any analog

input and a common terminal. One converter package

can now accommodate ground referenced 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. In the differential case, it also assigns the

polarity of the analog channels. Differential inputs are

restricted to adjacent channel pairs. For example, channel 0

and channel 1 may be selected as a different pair but

channel 0 or channel 1 cannot act differentially with any

other channel. In addition to selecting the differential mode,

the sign 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 illustrated by the MUX

addressing codes shown in Tables 1, 2, and 3.

The MUX address is shifted into the converter via the DI

input. Since the ML2281 contains only one differential

input channel with a fixed polarity assignment, it does

not require addressing.

The common input line on the ML2288 can be used as a

pseudo differential input. In this mode, the voltage on the

COM 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 applications where the analog circuitry may be

biased at a potential other than ground and the output

signals are all referred to this potential.

Since the input configuration is under software control, it

can be modified, as required, at each conversion. A channel

can be treated as a single-ended, ground referenced input

for one conversion; then it can be reconfigured as part of a

differential channel for another conversion. Figure 7

illustrates these different input modes.

SINGLE-ENDED MUX MODE

MUX ADDRESS ANALOG SINGLE-ENDED CHANNEL#

SGL/ ODD/ SELECT

DIF SIGN 1 0 0 1 2 3 4 5 6 7 COM

1

0 0 0+

–

1

0 01

+

–

1

0 10

+

–

1

0 11

+

–

1

1 00

+

–

1

1 01

+

–

1

1 10

+

–

1

1 11

+–

DIFFERENTIAL MUX MODE

MUX ADDRESS

ANALOG DIFFERENTIAL

CHANNEL-PAIR#

SGL/ ODD/ SELECT

0

1

2

3

DIF SIGN 1 0 0 1 2 3 4 5 6 7

0

0 00 + –

0

0 01

+–

0

0 10

+–

0

0 11

+–

0

1 00 – +

0

1 01

–+

0

1 10

–+

0

1 11

–+

Table 1. ML2288 MUX Addressing 8 Single-Ended

or 4 Differential Channels

SINGLE-ENDED MUX MODE

MUX ADDRESS

SGL/ ODD/ SELECT

DIF SIGN

1

0

1

0

0

+

1

0

1

1

1

0

1

1

1

CHANNEL#

1

2

3

+

+

+

COM is internally tied to AGND

DIFFERENTIAL MUX MODE

MUX ADDRESS

CHANNEL#

SGL/ ODD/ SELECT

DIF SIGN

1

0

1

2

3

0

0

0

+

–

0

0

1

+

–

0

1

0

–

+

0

1

1

–

+

Table 2. ML2284 MUX Addressing 4 Single-Ended

or 2 Differential Channel

12

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