Datasheet

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OVERVIEW
INPUT MULTIPLEXER
BURNOUT CURRENT SOURCES
INPUT BUFFER
IDAC1 AND IDAC2
A
IN
3
A
IN
4
A
IN
5
A
IN
6
A
IN
0
A
IN
1
A
IN
2
A
IN
7
A
INCOM
Burnout Current Source On
Burnout Current Source On
IDAC1
AGND
AV
DD
TEMPERATURE SENSOR
ADS1218
SBAS187C – SEPTEMBER 2001 – REVISED SEPTEMBER 2005
open. The anode of the diode is connected to the
positive input of the A/D converter, and the cathode
of the diode is connected to negative input of the A/D
The input multiplexer provides for any combination of
converter. The output of IDAC1 is connected to the
differential inputs to be selected on any of the input
anode to bias the diode and the cathode of the diode
channels, as shown in Figure 25 . For example, if
is also connected to ground to complete the circuit.
channel 1 is selected as the positive differential input
In this mode, the output of IDAC1 is also connected
channel, any other channel can be selected as the
to the output pin, so some current may flow into an
negative differential input channel. With this method,
external load from IDAC1, rather than the diode. See
it is possible to have up to eight fully differential input
Application Report Measuring Temperature with the
channels.
ADS1256, ADS1217, or ADS1218 (SBAA073 ) for
In addition, current sources are supplied that will
more information.
source or sink current to detect open or short circuits
on the input pins.
When the Burnout bit is set in the ACR configuration
register, two current sources are enabled. The current
source on the positive input channel sources
approximately 2µA of current. The current source on
the negative input channel sinks approximately 2µA.
This allows for the detection of an open circuit
(full-scale reading) or short circuit (0V differential
reading) on the selected input differential pair.
The input impedance of the ADS1218 without the
buffer is 5M Ω /PGA. With the buffer enabled, the input
voltage range is reduced and the analog
power-supply current is higher. The buffer is
controlled by ANDing the state of the BUFEN pin with
the state of the BUFFER bit in the ACR register. See
Application Report Input Currents for High-Resolution
ADCs (SBAA090 ) for more information.
The ADS1218 has two 8-bit current output DACs that
can be controlled independently. The output current is
set with R
DAC
, the range select bits in the ACR
register, and the 8-bit digital value in the IDAC
register.
The output current = V
REF
/(8R
DAC
)(2
RANGE–1
)(DAC
CODE). With V
REFOUT
= 2.5V and R
DAC
= 150k Ω to
AGND the full-scale output can be selected to be
0.5mA, 1mA, or 2mA. The compliance voltage range
is 0V to within 1V of AV
DD
. When the internal voltage
Figure 25. Input Multiplexer Configuration
reference of the ADS1218 is used, it is the reference
for the IDAC. An external reference may be used for
the IDACs by disabling the internal reference and
tying the external reference input to the V
REFOUT
pin.
An on-chip diode provides temperature sensing
capability. When the configuration register for the
input MUX is set to all 1s, the diode is connected to
the input of the A/D converter. All other channels are
16