AN929 Datasheet, PDF (20/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
CONCLUSION
Temperature sensors are used in embedded systems
for both thermal monitoring and management applica-
tions. A designer must evaluate the trade-offs of the
sensor, conditioning circuitry and sensor output in
order to maximize the measurement accuracy while
easing the interface to the microcontroller. In addition,
the designer must consider system integration issues
such as the location of the sensor, grounding, EMI/ESD
protection and shielding in order to provide a robust
temperature measurement. A sample of practical
circuits and interface techniques has been provided
along with design equations.
The following sensor guidelines can be used as a start-
ing point to select a temperature sensor. If your appli-
cation requires a high-temperature measurement,
thermocouples are a good choice because of their wide
temperature operating range. Thermocouples are typi-
cally used as remote sensors and, therefore, the circuit
must provide noise immunity by using good grounding
and shielding methods. If your application requires pre-
cision, RTDs set the standard with their superior
repeatability and stability characteristics. For applica-
tions such as the temperature measurement on a PCB,
either thermistors or silicon IC sensors should be con-
sidered. Thermistors are available in more packages,
are lower in cost and have a faster thermal response
time than silicon sensors. However, thermistors require
additional signal-conditioning circuitry, while silicon
sensors provide both the sensor and circuitry on a
single IC that can be interfaced directly to the
microcontroller.
The output of the sensor is selected by the available
microcontroller hardware and software resources, in
addition to the complexity of the sensor circuit. The
sensor output can consist of an analog, frequency,
ramp rate, duty cycle, serial or logic format that is
proportional to temperature. Temperature measure-
ment is a popular topic and the designer should review
the literature to evaluate the many sensor and circuit
options available.
REFERENCES
1. AN679, “Temperature Sensing Technologies”,
DS00679, Baker, Bonnie, Microchip Technology
Inc., 1999.
2. “High-Accuracy CMOS Smart Temperature
Sensors”, Bakker, A. and Huijsing, J., Kluwer
Academic Publishers, Boston, 2000.
3. AN913, “Interfacing the TC77 Thermal Sensor
to a PICmicro^® Microcontroller”, DS00913,
Bible, S. and Lepkowski, J.,
Microchip Technology Inc., 2004.
4. AN897, “Thermistor Temperature Sensing with
the MCP6S2X PGA”, DS00897, Blake, K.,
Microchip Technology Inc., 2004.
5. AN512, “Implementing Ohmmeter/Temperature
Sensor”, DS00512, Cox, D.,
Microchip Technology Inc., 1997.
6. TB052, “Multi-Zone Temperature Monitoring
with the TCN75 Thermal Sensor”, DS91052,
Dietz, K., Microchip Technology Inc., 2001.
7. AN895, “Oscillator Circuits for RTD Temperature
Sensors”, DS00895, Haile, E. and Lepkowski,
J., Microchip Technology Inc., 2004.
8. “Section 7: Temperature Sensors, Practical
Design Techniques for Sensor Signal
Conditioning”, Kester, W., Bryant, W. and Jung,
W., Analog Devices, 1999.
9. AN871, “Solving Thermal Measurement
Problems Using the TC72 and TC77 Digital
Silicon Temperature Sensors”, DS00871,
Lepkowski, J., Microchip Technology Inc., 2003.
10. “Silicon Sensors Harness Thermal
Management”, Marsh, D., EDN,
December 11, 2003.
11. “NTC Thermistor Basics and Principles of
Operation”, McGillicuddy, D. Sensors,
December, 1993.
12. “The ABCs of RTDs”, McGovern, B., Sensors,
November 2003.
13. “Noise Reduction Techniques in Electronic
Systems”, Ott, H, John Wiley, N.Y., 1998.
14. AN571, “Using Analog Temperature Sensors
with ADCs”, Maxim Semiconductor, 2001.
ACKNOWLEGDEMENT
The author appreciates the assistance of Kumen Blake
of Microchip Technology. The thermistor circuit design
examples are based on his work.
DS00929A-page 20
 2004 Microchip Technology Inc.