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Texas Instruments Precision Thermocouple Measurement with the ADS1118 Application notes
Application Report
SBAA189 – September 2011
Precision Thermocouple Measurement with the ADS1118
Mike Beckman, Luis Chioye ............................................................................. High-Performance Analog
ABSTRACT
Thermocouples are a very common form of temperature measurement devices that are widely used in
both industrial and general-purpose applications. Traditionally, achieving high system accuracy from
thermocouple measurements can be difficult because there are many sources of error within a given
system. The ADS1118 is an analog-to-digital converter (ADC) from TI that offers a precision analog
front-end specifically designed for precise measurements of thermocouples. The ADS1118 makes
thermocouple designs simple, and also provides on-chip cold junction compensation. This application note
presents the basic principles of thermocouple operation as well as the real-world implications of these
principles. It reviews both design and layout considerations for achieving a high-accuracy, robust design.
Experimental results for ADS1118 thermocouple measurements and cold junction compensation are
discussed, as well as a recommended software flow that demonstrates how to simplify a lookup table by
using linear interpolation.
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2
3
4
Contents
Introduction to Thermocouples ............................................................................................
Design and Layout Considerations .......................................................................................
Software Flow ...............................................................................................................
Experimental Results .......................................................................................................
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4
6
8
List of Figures
1
2
3
4
5
6
7
8
9
10
11
12
13
.........................................................................................
Illustration of the Seebeck Effect..........................................................................................
Two-Channel Thermocouple System.....................................................................................
ADS1118EVM Bottom Side Layout .......................................................................................
ADS1118EVM Top Side Layout ..........................................................................................
Typical ADS1118 Thermocouple Application Component Placement ...............................................
Software Flow Block Diagram .............................................................................................
V-to-I Conversion Block Diagram .........................................................................................
Comparison of Interpolation Errors Using Various Lookup Tables ...................................................
Experimental Setup With Varying Cold Junction Temperature .......................................................
Thermocouple Accuracy with Varying Cold Junction Temperature on ADS1118 EVM ...........................
Experimental Setup with Varying Thermocouple ......................................................................
Experimental Temperature Error for Cold Junction Compensation on ADS1118EVM ...........................
Thermocouple Junction Diagram
2
2
4
5
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6
6
7
8
9
9
10
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Introduction to Thermocouples
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Introduction to Thermocouples
Thermocouples are a popular type of temperature measurement device. A relatively low price, wide
temperature range, lack of required excitation, long-term stability, and proficiency with contact
measurements make these devices very common in a wide range of applications. While achieving
extremely high accuracy with a thermocouple can be more difficult than a resistance temperature detector
(or RTD), the low cost and versatility of a thermocouple often make up for this difficulty in precision.
Additionally, in contrast with thermistors and RTDs, the use of thermocouples often simplifies application
circuitry because they require no excitation. That is, these devices generate voltage without any additional
active circuitry. Thermocouples do, however, require a stable voltage reference and some form of ice point
or cold junction compensation. The integration of an internal voltage reference, multiplexer, and
temperature sensor make the ADS1118 an ideal option for thermocouple measurements.
A thermocouple is a length of two wires made from two dissimilar conductors (usually alloys) that are
soldered or welded together at one end, as shown in Figure 1. The composition of the conductors used
varies widely, and depends on the required temperature range, accuracy, lifespan, and environment that is
being measured. However, all thermocouple types operate based on the same fundamental theory: the
thermoelectric or Seebeck effect. Whenever a conductor experiences a temperature gradient from one
end of the conductor to the other, a voltage potential develops. This voltage potential arises because free
electrons within the conductor diffuse at different rates, depending on temperature. Electrons with higher
energy on the hot side of the conductor diffuse more rapidly than the lower energy electrons on the cold
side. The net effect is that a buildup of charge occurs at one end of the conductor and creates a voltage
potential from the hot and cold ends. This effect is illustrated in Figure 2.
Metal 1
Metal 3
Junction A
Junction B
Metal 3
Metal 2
Junction C
Figure 1. Thermocouple Junction Diagram
V
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-
-
-
-
-
-
-
-
-
Hot
Cold
Figure 2. Illustration of the Seebeck Effect
2
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Introduction to Thermocouples
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Different types of metals exhibit this effect at varying levels of intensity. When two different types of metals
are paired together and joined at a certain point (junction A in Figure 1), the differences in voltage on the
end opposite of the short (junctions B and C) are proportional to the temperature gradient formed from
either end of the pair of conductors. The implication of this effect is that thermocouples do not actually
measure an absolute temperature; they only measure the temperature difference between two points,
commonly known as the hotand cold junctions. Therefore, in order to determine the temperature at either
end of a thermocouple, the exact temperature of the opposite end must be known.
In a classical design, one end of a thermocouple is kept in an ice bath (junctions B and C in Figure 1) in
order to establish a known temperature. In reality, for most applications, it is not practical to provide a true
ice point reference. Instead the temperature of junctions B and C of the thermocouple are continuously
monitored and used as a point of reference to calculate the temperature at junction A at the other end of
the thermocouple. These junctions are known as the cold junctions or ice point for historical reasons,
although they do not need to be kept cold or near freezing. These endpoints are referred to as junctions
because they connect to some form of terminal block that transitions from the thermocouple alloys into the
traces used on the printed circuit board, or PCB (usually copper). This transition back to copper is what
creates the cold junctions B and C. Because of the law of intermediate metals, junctions B and C can be
treated as a single reference junction, provided that they are held at the same temperature or isothermal.
Once the temperature of the reference junction is known, the absolute temperature at junction A can be
calculated. Measuring the temperature at junctions B and C and then using that temperature to calculate a
second temperature at junction A is known as cold junction compensation.
In many applications, the temperature of junctions B and C are measured using a diode, thermistor, or
RTD. Because of the high-accuracy, onboard temperature sensor, small size, and minimal signal path
requirements of the ADS1118, it is possible use the device to achieve cold junction compensation. As with
any form of cold junction compensation, it is important that two conditions are met to achieve accurate
thermocouple measurements:
• Junctions B and C must be kept isothermal or be held at the same temperature. This condition
can be achieved by keeping junctions B and C in very close proximity to each other and away from
any sources of heat that may exist on a PCB. Many times, isothermal blocks are used to keep the
junctions at the same temperature. A large mass of metal offers a very good form of isothermal
stabilization. For other applications, it may be sufficient to maximize the copper fill around the
junctions. By creating an island of metal fill on both top and bottom layers, joined with periodically
placed vias, a simple isothermal block can be created. It is important to ensure that this isothermal
block cannot be impacted by parasitic heat sources from other areas in the circuit, such as power
conditioning circuitry.
• The isothermal temperature of junctions B and C must be accurately measured. The closer that a
temperature sensor (such as a diode, RTD, or thermistor) can be placed to the isothermal block, the
better. This recommendation also applies to the ADS1118. With only 500 μW of power consumption,
the effects of the ADS1118 self-heating are negligible for most applications. At 0.5ºC maximum error,
the ADS1118 offers excellent uncalibrated precision, which can be further improved through a system
calibration. Air currents can also act to reduce the accuracy of the cold junction compensation
measurement. To achieve the best performance, it is recommended to ensure that the cold junction be
kept within an enclosure and that air currents be kept to a minimum near the cold junction. In
applications where air currents are unavoidable, it may be useful to find a mechanical method to cover
the ADS1118 and connector block in the form of some type of shielding that protects the cold junction
from air currents. It is also important to remember that the orientation of the PCB can impact the
accuracy of the cold junction compensation. If there are heat-generating elements physically below the
cold junction, for example, inaccuracies can become significant as heat from those elements rises.
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Design and Layout Considerations
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Design and Layout Considerations
Figure 3 illustrates the basic connections for an independent, two-channel thermocouple system. This
circuit contains a simple low-pass, anti-aliasing filter, mid-point bias, and open detection. While the digital
filter of the ADS1118 strongly attenuates high-frequency components of noise, providing a first-order,
passive RC filter to further improve this performance and avoid aliasing is a good practice. The differential
RC filter formed by the 500-Ω resistors (RDIFFA and RDIFFB) and the 1-µF capacitor (CDIFF) offers a cutoff
frequency of approximately 320 Hz. Additional filtering can be achieved by increasing the differential
capacitor or the resistance values. However, avoid increasing the filter resistance beyond 1 kΩ on each
input, because the effects of interaction with ADC input impedance will begin to affect the linearity and
gain error of the ADS1118. Also, as a result of the high sampling rates supported by the ADS1118, simple
post digital filtering with a microcontroller can alleviate the requirements of the analog filter, and also offer
the flexibility to implement filter notches at 50 Hz or 60 Hz. Two 0.1-µF capacitors (CCMA and CCMB) are
also added to offer attenuation of high-frequency, common-mode noise components.
Because mismatches in the common-mode capacitors cause differential noise, it is recommended that the
differential capacitor be at least an order of magnitude (10x) greater than the common-mode capacitors.
To achieve good electromagnetic interference (EMI) immunity, it is important to remember that simply
placing large capacitors in the signal path and supply are not effective at attenuating high noise frequency
components. Using small (10 nF and lower) capacitors with low equivalent series resistance (ESR) and
low dielectric absorption (DA) in parallel with another higher capacitance capacitor on sensitive supply and
signal paths can offer significant improvements to EMI immunity. Additional EMI protection can further be
realized by incorporating a ferrite bead or common-mode choke to the inputs. If there is significant
concern that there may be frequent exposure to electrical overstress or electrostatic discharge (ESD),
Schottky clamp diodes can be added to the exposed inputs before the input filter.
The two 1-MΩ resistors (RPU and RPD) serve two purposes. First, these components offer a common-mode
bias near midsupply. Connecting only 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 these 1-MΩ resistors is to offer a weak pull-up and pull-down for sensor open detection. If a
sensor is disconnected, the inputs to the ADC will extend to supply and ground and yield a full-scale
readout that indicates a sensor disconnection. For extremely long thermocouples, these 1-MΩ resistors
may impact measurement accuracy. Increasing the resistance can alleviate these effects. Alternatively,
two 1-MΩ resistors connected as a resistor divider to one of the inputs can maintain the midpoint bias
without affecting measurement results. This method, however, sacrifices open detection and a small
amount of common-mode noise rejection.
3.3V
GND
3.3V
RPU = 1Mȍ
0.01 F
GND
GND
CCMA = 0.1 F
500ȍ
AIN0
VDD
RDIFFA
CDIFF = 1 F
RDIFFB
RPD = 1Mȍ
0.1 F
500ȍ
AIN1
(PGA Gain = 16)
±256mV FS
CCMB = 0.1 F
Voltage
Reference
ADS1118
SCLK
GND
GND
PGA
GND
3.3V
RPU = 1Mȍ
MUX
16-bit
Ȉǻ
ADC
Digital Filter
and
Interface
DIN
CCMA = 0.1 F
500ȍ
CS
DOUT/DRDY
AIN2
RDIFFA
CDIFF = 1 F
RDIFFB
RPD = 1Mȍ
GND
500ȍ
Oscillator
AIN3
Temp Sensor
GND
CCMB = 0.1 F
GND
GND
Figure 3. Two-Channel Thermocouple System
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Design and Layout Considerations
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Because the accuracy of the overall temperature sensor depends on how accurately the ADS1118 can
measure the cold junction, careful PCB layout considerations must be employed when designing an
accurate thermocouple system. The ADS1118EVM provides a good starting point and offers an example
of one way to achieve good cold junction compensation performance. The ADS1118EVM uses the same
schematic shown in Figure 3, except with only one thermocouple channel connected. The layout for the
evaluation module (EVM) is shown in Figure 4 and Figure 5. In the layout diagram in Figure 4, C10
corresponds to CDIFF, C2 corresponds to CCMA, C3 corresponds to CCMB, R3 corresponds to RDIFFA, R4
corresponds to RDIFFB, R1 corresponds to RPU and R2 corresponds to RPD.
Figure 4. ADS1118EVM Bottom Side Layout
Solid Metal Fill
Ground Plane
Solid Metal Fill
Ground Plane
ADS1118
Thermocouple
Connector
Solid Metal Fill
Ground Plane
Figure 5. ADS1118EVM Top Side Layout
The ADS1118EVM layout is designed in a modular way that allows an interfacing board with a
microcontroller. In an actual system, a microcontroller, power conditioning, and some form of interface
transceiver is likely to be present. In order to achieve optimal noise and thermal performance, it is
important to isolate the ADS1118 away from digital components as well as any heat-generating
components. Because there are no digital or heat-generating ICs on the ADS1118EVM, there is very little
error because of noise and parasitic thermal gradients on the board. However for many systems, careful
consideration regarding the parasitic heat generated by other components should be carefully considered
when performing system layout.Figure 6 shows a good component placement diagram for thermocouple
systems using the ADS1118 in addition to commonly-used components within a typical thermocouple
system. Notice that the ADS1118 is kept as close to the thermocouple connection as possible. Also note
that there is a ground fill around the device and connector.
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Software Flow
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In the example of Figure 6, several vias are shown that connect to another ground fill on the other side of
the board. Having an additional layer helps to improve the temperature consistency of the board. The
metal fill not only conducts the temperature of the cold junction to the ADS1118 very well, it also helps
ensure that both junctions are kept isothermal. Furthermore, there is a ground fill cut that isolates all other
active components from the ADS1118 and the thermocouple cold junction. This layout helps avoid
parasitic heat transfer from other active components in the system and can greatly improve noise
performance.
Ground fill or
Ground plane
Ground fill or
Ground plane
Ground fill cut
Supply
Generation
RC Filter
Through hole via
Interface
Tranceiver
Microprocessor
ADS1118
Connector or
Antenna
Ground fill or
Ground plane
Ground fill or
Ground plane
Ground fill cut
Figure 6. Typical ADS1118 Thermocouple Application Component Placement
3
Software Flow
The calculation procedure to achieve cold junction compensation is simple and can be done in several
ways. One typical 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, in turn, will yield two results for every thermocouple measurement and cold
junction measurement cycle: the thermocouple voltage or VTC, and the on-chip temperature or TCJC. In
order to account for the cold junction, the temperature sensor within the ADS1118 must first be converted
to a voltage that is proportional to the thermocouple currently being used, to yield VCJC. This process is
generally accomplished by performing a reverse lookup on the table used for the thermocouple
voltage-to-temperature conversion. Adding the two voltages then yields the thermocouple-compensated
voltage VActual, where VCJC + VTC = VActual. VActual is then converted to temperature using the same lookup
table from before, and yields TActual. A block diagram showing this process is given in Figure 7
ADS1118
Thermocouple
Voltage
MCU
VTC
TActual
On-chip
Temperature
TCJC
T
V
VCJC
Ȉ
VActual
V
T
Result
Figure 7. Software Flow Block Diagram
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Software Flow
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The conversion from thermocouple temperature to voltage and voltage to temperature can be performed
in two ways. First, the coefficients can be programmed into the microcontroller from the high-order
polynomial, and then the calculation can be performed on each reading. While this method offers the
smallest introduced error during the conversion, it is extremely processor-intensive and is not practical for
most applications. The second and most common way to perform the conversion is through the use of a
lookup table. Thermocouple manufacturers usually provide a lookup table with their respective
thermocouple devices that offer excellent accuracy for linearization of a specific type of thermocouple. The
granularity on these lookup tables is also very precise—approximately 1°C for each lookup value. To save
microcontroller memory and development time, an interpolation technique applied to these values can be
used. An example of this method when converting from voltage to temperature with eight look-up table
entries is shown in Figure 8.
Voltage
Lookup Table
V
Increment pointer
until lookup table
voltage exceeds
input voltage
Ex. Vin = 9.01mV
Temperature
Lookup Table
0
0mV
0°C
1
2.96mV
72.61°C
2
5.9mV
144.12°C
n-1
3
8.86mV
217.99°C
n
4
11.8mV
290.15°C
5
14.74mV
360.64°C
6
17.7mV
430.78°C
7
20.65mV
500°C
Perform
interpolation using
Equation 1
T
Figure 8. V-to-I Conversion Block Diagram
To perform a linear interpolation using a lookup table, first compare the value that must be converted to
values in the lookup table, until the lookup table value exceeds the value that is being converted. Then,
use Equation 1 to convert to temperature, where VLT is the voltage lookup table array and TLT is the
temperature lookup table array. This operation involves four additions, one multiplication, and one division
step, respectively. This operation can be done easily on most 16- and 32-bit microcontrollers. Converting
from temperature to voltage is the same, except that the lookup tables and the temperature variables are
reversed, as shown in Equation 2.
T TLT [n 1] (TLT [n] TLT [n 1])
V VLT [n 1] (VLT [n] VLT [n 1])
VIN VLT [n 1]
VLT [n] VLT [n 1]
(1)
TIN TLT [n 1]
TLT [n] TLT [n 1]
(2)
The number of entries used for a lookup table will affect the accuracy of the conversion. For the majority
of applications, 16 to 32 lookup table entries should be sufficient. Also, the lookup table entries do not
need to equally spaced. By carefully placing them in highly nonlinear portions of the thermocouple transfer
functions, the number of required lookup table entries can be minimized. Furthermore, they also do not
need to be incorporated in powers of 2, as shown in the examples within this document.
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Experimental Results
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Figure 9 shows the conversion error that can be expected from linear interpolation using a lookup table for
a K-type thermocouple from 0ºC to +500ºC. Because the number of lookup table entries exceeds 16, the
improvement in accuracy become smaller and smaller.
Linear Interpolation Temperature Error (°C)
1.5
4 Lookup Table Entries
8 Lookup Table Entries
16 Lookup Table Entries
32 Lookup Table Entries
1
0.5
0
−0.5
−1
−1.5
0
50
100
150
200
250
300
Thermocouple Temperature (°C)
350
400
450
500
G000
Figure 9. Comparison of Interpolation Errors Using Various Lookup Tables
4
Experimental Results
An excellent way to test the accuracy of the on-chip cold junction compensation with the ADS1118 is to
place the ADS1118 and cold junction into a temperature-controlled environment, and place the other end
of the thermocouple into a known constant temperature source such as a thermal bath. This experimental
setup is shown in Figure 10. When performing this experiment, it is best to try and mimic the actual
environment in which the system board is to be used. If the ADS1118 and cold junction are within an
enclosure that does not have significant air currents present, a simple oven should be sufficient. However,
in applications that must endure high air currents, a temperature-forcing system with aggressive air
currents may be useful to benchmark the system performance. The accuracy of the oven or
temperature-forcing system used on the ADS1118 system board and cold junction does not need to be
highly accurate. The other end of the thermocouple, however, must be held at a very constant and
accurate temperature. One of the best ways to achieve this constant temperature is by using a thermal
bath or a well-insulated bath of ice water.
In order to perform this experiment, the temperature of the ADS1118 PCB and thermocouple cold junction
is swept, while the temperature of the end of the thermocouple is held constant in the thermal bath. The
temperature measurements of the thermocouple are recorded and plotted against the cold junction
temperature (oven temperature).
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Figure 10 shows the set-up using the ADS1118EVM board and a K-type thermocouple. The SM-USB-DIG
Platform and USB cable remain outside the oven.
88.8°C
25°C
Varying from
-40°C to +125°C
Fixed at 25°C
Figure 10. Experimental Setup With Varying Cold Junction Temperature
Figure 11 shows the plot of the thermocouple measurements against the cold junction temperature
obtained in this experiment. This setup is intended to reveal inaccuracies that arise because of changes to
the system board temperature, cold junction temperature, and ADS1118 temperature. The results indicate
an approximately 0.4ºC drift when the system is drifted from 0ºC to +70ºC and around 0.9ºC variation over
the complete specified temperature range of –40ºC to +125ºC for the ADS1118. These results were
obtained with a factory-trimmed ADS1118 with no additional calibrations, and include all errors as a result
of ADS1118 internal reference drift, internal temperature sensor error, and isothermal errors.
26
Thermocouple Temperature (°C)
25.8
Actual Thermocouple Temperature
Thermocouple Result from ADS1118 with CJC
25.5
25.2
25
24.8
24.5
24.2
24
−40
−30
−20
−10
0
10
20
30
40
50
60
70
Cold Junction Temperature (°C)
80
90
100
110
120
130
G000
Figure 11. Thermocouple Accuracy with Varying Cold Junction Temperature on ADS1118 EVM
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Experimental Results
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A second experiment that tests the overall system performance by verifying the accuracy of the
thermocouple temperature measurement system is to sweep the temperature of the thermocouple using a
very stable, uniform, and accurate temperature source. A calibrated thermal bath is a good temperature
source for this test. The setup for this test is shown in Figure 12. This experiment is specifically intended
to reveal inaccuracies in the thermocouple itself and any errors that occur because of the analog-to-digital
conversion process. The cold junction is held at a relatively constant ambient temperature (room
temperature) with no temperature forcing. The results in Figure 13 indicate approximately 1.5ºC of error
from –40ºC to +150ºC. This result is well within the accuracy limitations of the K-type thermocouple used
(also included with the ADS1118EVM), which is specified to be accurate to within 2.2°C. More precise
tests can be performed using a calibrated thermocouple.
25°C
Figure 12. Experimental Setup with Varying Thermocouple
2
Thermocouple Temperature Error (°C)
K−type thermocouple used is rated to ±2.2°C max error
1.5
1
0.5
0
−0.5
−1
−1.5
−2
−40
−30
−20
−10
0
10
20
30
40
50
60
70
80
90
Actual Thermocouple Temperature (°C)
100
110
120
130
140
150
G000
Figure 13. Experimental Temperature Error for Cold Junction Compensation on ADS1118EVM
The results shown in Figure 11 and Figure 13 are typical results using the ADS1118EVM and the
thermocouple provided with the EVM. The actual performance in a given system may be different and
depends on many variables, including (but not limited to) the application schematics, PCB layout,
temperature-forcing system accuracies, and environmental noise contributions, among other factors. TI
offers no assurance of system performance other than the performance parametrics detailed in the
Electrical Characteristics section of the ADS1118 product data sheet.
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specifically designated by TI as military-grade or "enhanced plastic." Only products designated by TI as military-grade meet military
specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at
the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are
designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated
products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Products
Applications
Audio
www.ti.com/audio
Communications and Telecom www.ti.com/communications
Amplifiers
amplifier.ti.com
Computers and Peripherals
www.ti.com/computers
Data Converters
dataconverter.ti.com
Consumer Electronics
www.ti.com/consumer-apps
DLP® Products
www.dlp.com
Energy and Lighting
www.ti.com/energy
DSP
dsp.ti.com
Industrial
www.ti.com/industrial
Clocks and Timers
www.ti.com/clocks
Medical
www.ti.com/medical
Interface
interface.ti.com
Security
www.ti.com/security
Logic
logic.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Power Mgmt
power.ti.com
Transportation and Automotive www.ti.com/automotive
Microcontrollers
microcontroller.ti.com
Video and Imaging
RFID
www.ti-rfid.com
OMAP Mobile Processors
www.ti.com/omap
Wireless Connctivity
www.ti.com/wirelessconnectivity
TI E2E Community Home Page
www.ti.com/video
e2e.ti.com
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2011, Texas Instruments Incorporated
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