ADS1218_14 Datasheet, PDF (16/45 Pages) Texas Instruments – 8-Channel, 24-Bit ANALOG-TO-DIGITAL CONVERTER with FLASH Memory

ADS1218
SBAS187C – SEPTEMBER 2001 – REVISED SEPTEMBER 2005
www.ti.com
OVERVIEW
INPUT MULTIPLEXER
The input multiplexer provides for any combination of
differential inputs to be selected on any of the input
channels, as shown in Figure 25. For example, if
channel 1 is selected as the positive differential input
channel, any other channel can be selected as the
negative differential input channel. With this method,
it is possible to have up to eight fully differential input
channels.
In addition, current sources are supplied that will
source or sink current to detect open or short circuits
on the input pins.
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
converter. The output of IDAC1 is connected to the
anode to bias the diode and the cathode of the diode
is also connected to ground to complete the circuit.
In this mode, the output of IDAC1 is also connected
to the output pin, so some current may flow into an
external load from IDAC1, rather than the diode. See
Application Report Measuring Temperature with the
ADS1256, ADS1217, or ADS1218 (SBAA073) for
more information.
BURNOUT CURRENT SOURCES
A_IN0
A_IN1
AV_DD
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.
Burnout Current Source On
A_IN2
INPUT BUFFER
The input impedance of the ADS1218 without the
A_IN3
buffer is 5MΩ/PGA. With the buffer enabled, the input
voltage range is reduced and the analog
power-supply current is higher. The buffer is
A_IN4
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
A_IN5
ADCs (SBAA090) for more information.
A_IN6
A_IN7
A_INCOM
Burnout Current Source On
AGND
IDAC1
Figure 25. Input Multiplexer Configuration
TEMPERATURE SENSOR
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
IDAC1 AND IDAC2
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 = 150kΩ 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
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.
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