ADC0838 Micro Linear Corporation, ADC0838 Datasheet - Page 12

• Manufacturer Part Number: ADC0838
• Description: Serial I/O 8-Bit A/D Converters with Multiplexer Options
• Manufacturer: Micro Linear Corporation

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.
12
SINGLE-ENDED MUX MODE
DIFFERENTIAL MUX MODE
SINGLE-ENDED MUX MODE
DIFFERENTIAL MUX MODE
SGL/ ODD/
SGL/ ODD/
SGL/ ODD/
SGL/ ODD/
DIF
DIF
DIF
DIF
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
1
0
0
0
0
Table 1. ML2288 MUX Addressing 8 Single-Ended
Table 2. ML2284 MUX Addressing 4 Single-Ended
M
M
M
M
UX ADDRESS
UX ADDRESS
UX ADDRESS
UX ADDRESS
SIGN
SIGN
SIGN
SIGN
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
SELECT
SELECT
or 4 Differential Channels
SELECT
SELECT
or 2 Differential Channel
1
0
0
1
1
0
0
1
1
1
0
0
1
1
0
0
1
1
1
0
1
0
1
1
0
1
0
1
0
0
1
0
1
0
1
0
1
0
0
1
0
1
0
1
0
1
ANALOG SINGLE-ENDED CHANNEL#
0
+
0
+
–
+
+
+
1
0
0
0
–
ANALOG DIFFERENTIAL
1
–
+
+
2
CHANNEL-PAIR#
+
2
–
3
+
CHANNEL#
CHANNEL#
COM is internally tied to AGND
+
+
1
1
1
–
4
+
3
+
–
+
5
+
4
–
2
2
+
2
+
–
+
6
5
+
–
7
+
+
6
–
COM
3
3
+
3
–
+
–
–
–
–
–
–
–
–
7
+
–