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
11
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