Rainbow Electronics ADC0838 - Manual (Page 11)

Functional Description
1 0 MULTIPLEXER ADDRESSING
The design of these converters utilizes a sample-data com-
parator structure which provides for a differential analog in-
put 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 being
converted indicates which line the converter expects to be
the most positive If the assigned ‘‘a’’ input is less than the
‘‘b’’ input 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 a new pseudo-differential op-
tion which will convert the difference between the voltage at
any analog input and a common terminal 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 In the differential case it also assigns the polarity of the
channels Differential inputs are restricted to adjacent chan-
nel pairs For example channel 0 and channel 1 may be
selected as a different pair but channel 0 or 1 cannot act
differentially with any other channel In addition to selecting
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 best
illustrated by the MUX addressing codes shown in the fol-
lowing tables for the various product options
The MUX address is shifted into the converter via the DI
line Because the ADC0831 contains only one differential
input channel with a fixed polarity assignment it does not
require addressing
The common input line on the ADC0838 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-supply appli-
cation 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 Multiplexer Package Options
Part Number of Analog Channels Number of
Number
Single-Ended Differential
Package Pins
ADC0831 1 1 8
ADC0832 2 1 8
ADC0834 4 2 14
ADC0838 8 4 20
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