Functional description – Rainbow Electronics ADC08138 User Manual
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Functional Description
1 0 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 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 ‘‘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 circuity may be biased up
to a potential other than ground and the output signals are
all referred to this potential
TABLE I Multiplexer Package Options
Part
Number of Analog Channels
Number of
Number
Single-Ended
Differential
Package Pins
ADC08131
1
1
8
ADC08134
4
2
14
ADC08138
8
4
20
TABLE II MUX Addressing ADC08138
Single-Ended MUX Mode
MUX Address
Analog Single-Ended Channel
START
SGL
ODD
SELECT
0
1
2
3
4
5
6
7
COM
DIF
SIGN
1
0
1
1
0
0
0
a
b
1
1
0
0
1
a
b
1
1
0
1
0
a
b
1
1
0
1
1
a
b
1
1
1
0
0
a
b
1
1
1
0
1
a
b
1
1
1
1
0
a
b
1
1
1
1
1
a
b
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