OVERVIEW
INPUT MULTIPLEXER
The input multiplexer provides for any combination of differ-
ential inputs to be selected on any of the input channels, as
shown in Figure 1. For example, if A
IN
0 is selected as the
positive differential input channel, any other channel can be
selected as the negative terminal for the differential input
channel. With this method, it is possible to have up to seven
single-ended input channels or four independent differential
input channels for the ADS1243, and three single-ended
input channels or two independent differential input channels
for the ADS1242.
The ADS1242 and ADS1243 feature a single-cycle settling
digital filter that provides valid data on the first conversion
after a new channel selection. In order to minimize the
settling error, synchronize MUX changes to the conversion
beginning, which is indicated by the falling edge of
DRDY
.
In
other words, issuing a MUX change through the WREG
command immediately after
DRDY
goes LOW minimizes the
settling error. Increasing the time between the conversion
beginning (DRDY goes LOW) and the MUX change com-
mand (t
DELAY
) results in a settling error in the conversion
data, as shown in Figure 2.
A
IN
0/D0
A
IN
1/D1
V
DD
A
IN
2/D2
Burnout Current Source
BURNOUT CURRENT SOURCES
A
IN
3/D3
A
IN
4/D4
Input
Buffer
A
IN
5/D5
The Burnout Current Sources can be used to detect sensor
short-circuit or open-circuit conditions. Setting the Burnout
Current Sources (BOCS) bit in the SETUP register activates
two 2µA current sources called burnout current sources. One
of the current sources is connected to the converter’s nega-
tive input and the other is connected to the converter’s
positive input.
Figure 3 shows the situation for an open-circuit sensor. This
is a potential failure mode for many kinds of remotely con-
nected sensors. The current source on the positive input acts
as a pull-up, causing the positive input to go to the positive
analog supply, and the current source on the negative input
acts as a pull-down, causing the negative input to go to
ground. The ADS1242/43 therefore outputs full-scale (7FFFFF
Hex).
Figure 4 shows a short-circuited sensor. Since the inputs are
Burnout Current Source
A
IN
6/D6
GND
A
IN
7/D7
ADS1243
Only
FIGURE 1. Input Multiplexer Configuration.
New Conversion Begins,
Previous Conversion Data
Complete Previous Conversion
New Conversion Complete
DRDY
t
DELAY
SCLK
DIN
MSB
LSB
SETTLING ERROR vs DELAY TIME
f
CLK
= 2.4576MHz
10
1
Settling Error (%)
0.1
0.01
0.001
0.0001
0.00001
0.000001
0
2
4
6
8
10
12
Delay Time, t
DELAY
(ms)
14
16
FIGURE 2. Input Multiplexer Configuration.
10
ADS1242, 1243
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