Texas Instruments | Methods to Calibrate Temperature Monitoring Systems | Application notes | Texas Instruments Methods to Calibrate Temperature Monitoring Systems Application notes

Texas Instruments Methods to Calibrate Temperature Monitoring Systems Application notes
Methods to Calibrate Temperature Monitoring Systems
Introduction
Temperature sensors are affected by various
environmental effects aside from manufacturing
process variations. These include thermal stress,
mechanical stress, radiation, humidity, and aging
during storage, shipment, and/or assembly that may
alter the device’s intrinsic characteristics (for example,
accuracy and reliability) after it is implemented on the
final system. Note that physical placement of the
temperature sensor has a significant impact on the
device’s apparent accuracy relative to the target heat
source due to the local temperature gradient. There is
a distinction between apparent and intrinsic accuracy.
Apparent accuracy can be improved with physical
design (for example, PCB design with improved heat
transfer characteristics) but intrinsic accuracy is an
inherent device characteristic. In addition, external
components connected to the temperature sensors (for
example, ADC and filters) may have a significant
impact on the overall system’s intrinsic performance.
Both environmental and system electrical factors may
require system calibration to achieve traceable system
accuracy.
thermistor applications will require a bias resistor,
which will introduce an addition source of error. The
system errors arising from the factors discussed
typically manifest themselves as system gain and
offset errors, which can be reduced to some extent
using calibration. For non-linear systems, an additional
linearization step may be required depending on the
application. The general 3-step process is shown in
Figure 2. Note that the figure depicts only the averagevalue lines. Actual sensor output will have a statistical
distribution about the average.
Figure 2. Generalize Three Steps of Temperature
Sensor Calibration
System Calibration for Traceability
The system calibration process allows the final
assembled measurement system to be compared to a
known, traceable measurement standard (for example,
the NIST, UL, EN, and so forth) to establish
quantifiable measurement uncertainty. In the ideal
case, the system response is linear and the system
can be easily calibrated with just a simple offset, or
gain and offset corrections. However, temperature
sensors are not perfectly linear and thus cannot be
easily calibrated without linearization. Typically, a nonlinear system response requires multipoint linearization
using a LUT prior to gain and offset calibration.
Figure 1. Example Temperature Sensing Circuit
With a Thermistor
Analog temperature sensors, such as the TMP236 or
NTC thermistors, require an analog-to-digital converter
(ADC) to translate voltage to temperature. This affects
overall system performance due to error contributions
from the ADC. An example thermistor circuit is shown
in Figure 1. Unlike an IC temperature sensor, some
SNOAA10 – December 2018
Submit Documentation Feedback
Analog Temperature Monitoring Systems
Analog temperature monitoring systems generally
require linearization and calibration to achieve a high
level of accuracy and traceability. The degree of
linearization depends on the linearity of the sensor
itself. NTC thermistors, for example, will generally
require more system tradeoffs (for example, memory,
CPU cycle, and sensitivity) for linearization compared
to analog IC temperature sensors (for example, the
Methods to Calibrate Temperature Monitoring Systems
Copyright © 2018, Texas Instruments Incorporated
1
www.ti.com
TMP236). Specifically, analog IC temperature sensors
are typically more linear across a wide temperature
range compared to NTC thermistors. Regardless, an
additional step of calibration is required to achieve
system-level accuracy that is traceable. For a more indepth discussion on linearization, refer to the
document listed in Table 2.
V+
ADD0
SCL
Serial
Interface
SDA
Register
Bank
ALERT
EEPROM
Calibration Methods
Oscillator
For production purposes, some system calibration is
performed on a statistically significant number of
systems (for example, 30) to determine the
appropriate correction coefficients for the total number
of systems. This statistical approach limits production
cost. In some cases, calibration methods are
performed at the production test stage using a singlepoint room temperature calibration. Multipoint
calibration at the production test stage can result in
better system accuracy but is more expensive to
perform. Thus, multipoint calibration processes are
typically applied to specialized systems where the
production volume is relatively low. Regardless of the
calibration method, the reference probe accuracy and
traceability are essential components of calibration.
Zero Calibration Sensors
Unlike analog temperature sensors, TI digital
temperature sensors such as TMP117 do not require
any additional system linearization or calibration to
achieve traceable system accuracy. As shown in
Figure 3, a digital sensor is effectively a temperature
monitoring system on a chip. These traceable devices
are linearized and calibrated in production and this
greatly simplifies system implementation. Note that the
TMP117 does feature an offset register to enable the
end user to calibrate any temperature offset for their
system (for example, from physical system
temperature gradient).
Control
Logic
Internal
Thermal
BJT
Temperature
Sensor
Circuitry
ADC
GND
Copyright © 2017, Texas Instruments Incorporated
Figure 3. TMP117 Functional Block Diagram
TI Temperature Sensors and Design Tips
To learn more about some of TI temperature sensors,
refer to Table 1. The key optimized parameters as well
as their trade-offs are listed. To learn more about PCB
guidelines, ambient air measurement, or linearization,
refer to Table 2.
Table 1. Device Recommendations
Device
Optimized Parameters
Performance Trade-Off
TMP117
Zero calibration and
linearization, high
accuracy
May have longer read
time compared to analog
sensors
TMP236
Linear analog output
without external bias
circuit
May require ADC
TMP390
Zero calibration and
linearization, integrated
temperature switch
Less features than digital
sensors
TMP61
2-pin, small package,
linear resistance
May require ADC and
bias circuit
Table 2. Related Documentation
2
Literature Number
Title
SNOA967
Temperature Sensors: PCB Guidelines for
Surface Mount Devices
SNOA966
TMP116 Ambient Air Temperature
Measurement
SNOAA12
Methods to Reduce Thermistor
Linearization Error, Memory, and Power
Requirements Over Wide Operating
Temperature Ranges
Methods to Calibrate Temperature Monitoring Systems
Copyright © 2018, Texas Instruments Incorporated
SNOAA10 – December 2018
Submit Documentation Feedback
IMPORTANT NOTICE AND DISCLAIMER
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE
DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS”
AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY
IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD
PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate
TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable
standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you
permission to use these resources only for development of an application that uses the TI products described in the resource. Other
reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third
party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims,
damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on
ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable
warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2018, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising