ADS1118_13 Datasheet, PDF (30/38 Pages) Texas Instruments – Ultra-Small, Low-Power, SPI™-Compatible, 16-Bit Analog-to-Digital Converter and Temperature Sensor with Internal Reference

ADS1118
SBAS457C – OCTOBER 2010 – REVISED FEBRUARY 2013
www.ti.com
The two 1-MΩ resistors (R_PU and R_PD) serve two purposes. The first purpose is to offer a common-mode bias
near midsupply. Although the ADS1118 does offer the ability to float the common-mode of a signal or connect
any of the inputs to a common point such as ground or supply, TI generally recommends avoiding such
situations. Connecting one of the inputs to a common point decreases performance by converting common-mode
noise into differential signal noise that is not strongly attenuated. The second purpose of the 1-MΩ resistors is to
offer a weak pull-up and pull-down for sensor open detection. In the event that a sensor is disconnected, the
inputs to the ADC extend to supply and ground and yield a full-scale readout, indicating a sensor disconnection.
The procedure to actually achieve cold-junction compensation is simple and can be done in several ways. One
way is to interleave readings between the thermocouple inputs and the temperature sensor. That is, acquire one
on-chip temperature result for every thermocouple ADC voltage measured. If the cold junction is in a very stable
environment, more periodic cold junction measurements may be sufficient. These operations yield two results for
every thermocouple measurement and cold junction measurement cycle: the thermocouple voltage V_TC and the
on-chip temperature T_CJC. In order to account for the cold junction, the temperature sensor within the ADS1118
must first be converted to a voltage proportional to the thermocouple currently being used yielding V_CJC. This
conversion is generally accomplished by performing a reverse lookup on the table being used for the
thermocouple voltage to temperature conversion. Then, adding the two voltages yields the thermocouple
compensated voltage (V_Actual), where V_CJC + V_TC = V_Actual. V_Actual is then converted to temperature using the
same lookup table as before, yielding T_Actual.
Thermocouple manufacturers usually supply a lookup table with their thermocouples that offer excellent accuracy
for linearization of a specific type of thermocouple. The granularity on these lookup tables is generally very
precise (at around 1°C for each lookup value). To save microcontroller memory and development time, an
interpolation technique applied to these values can be used. By choosing 16 to 32 equally-spaced values from
the manufacturer's lookup tables over a desired temperature range, using a simple linear approximation of
intervals between is generally very precise.
30
Submit Documentation Feedback
Product Folder Links: ADS1118
Copyright © 2010–2013, Texas Instruments Incorporated