Datasheet

ADS1240, 1241
10
SBAS173F
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channel. With this method, it is possible to have up to eight
single-ended input channels or four independent differential
input channels for the ADS1241, and four single-ended input
channels or two independent differential input channels for
the ADS1240. Note that A
INCOM
can be treated as an input
channel.
The ADS1240 and ADS1241 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.
BURNOUT CURRENT SOURCES
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 ADS1240/41 therefore outputs full-scale (7FFFFF
Hex).
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
A
IN
3/D3
A
IN
4/D4
A
IN
5/D5
A
IN
6/D6
A
IN
0/D0
A
IN
1/D1
A
IN
2/D2
A
IN
7/D7
A
INCOM
Burnout Current Source
Burnout Current Source
AGND
AV
DD
ADS1241
Only
Input
Buffer
FIGURE 1. Input Multiplexer Configuration.
FIGURE 2. Input Multiplexer Configuration.
SETTLING ERROR vs DELAY TIME
f
CLK
= 2.4576MHz
Delay Time, t
DELAY
(ms)
Settling Error (%)
2 4 6 8 10 12 14 160
10.000000
1.000000
0.100000
0.010000
0.001000
0.000100
0.000010
0.000001
New Conversion Begins,
Complete Previous Conversion
New Conversion Complete
t
DELAY
MSB LSB
DRDY
DIN
SCLK
(POL = 0)
Previous Conversion Data