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Manny Soltero

Aug 26, 2015

temperature
sensing blog

sensing

temperature
sensing

what
are you sensing

LM35

Temperature
Sensors

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Related

What are you sensing? Pros and cons of four temperature sensor
types

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Choosing temperature-sensing
products may seem trivial, but with the wide variety of products available,
this task can be quite daunting. In this blog post, I’ll present four types of
temperature sensors – resistance temperature detectors (RTDs), thermocouples,
thermistors and integrated circuit (IC) sensors with digital and analog
interfaces – and discusses the pros and cons of each.

From a system-level standpoint, the right temperature sensor for your
application will depend on the required temperature range, accuracy, linearity,
solution cost, features, power consumption, solution size, mounting (surface
mount vs. through-hole vs. off-board) and ease of designing the necessary
support circuitry.

RTDs

When measuring the resistance of an RTD while varying its temperature, the
response is almost linear, behaving like a resistor. As shown in Figure 1, the
RTD’s resistance curve is several degrees out of linearity (with a straight line
shown for reference) but is highly predictable and repeatable. To compensate for
this slight nonlinearity, most designers digitize the measured resistance value
and use a lookup table within the microcontroller to apply correction factors.
This repeatability and stability over a wide temperature range (roughly -250°C
to +750°C) makes RTDs useful in high-precision applications, including measuring
temperature of fluid or gas in pipes and tanks.

Figure 1: RTD resistance versus
temperature

The complexity of the circuitry to process the RTD analog signal varies
substantially based on the application. Components such as amplifiers and
analog-to-digital converters (ADCs), which generate their own inaccuracies, are
necessary. You can also achieve low-power operation by powering the sensor only
when a measurement is necessary, but this complicates the circuitry even more.
Plus, the power required to energize the sensor also raises its internal
temperature, which affects measurement accuracy. With only a few milliamps of
current, this self-heating effect can develop temperature errors that are
correctable but require further consideration. Also, keep in mind that the cost
of a wire-wound platinum or thin-film RTD can be relatively high, especially
when compared to an IC sensor.

Thermistors

A thermistor is another type of resistive sensor. There are a wide variety of
thermistors available, from inexpensive to high-precision products. Low-cost,
low-precision thermistors perform simple measurement or threshold-detection
functions; require multiple components (such as a comparator, reference and
discrete resistors); are very inexpensive; and have nonlinear
resistance-temperature properties, as shown in Figure 2. If you need to measure
a wide range of temperatures, you will need substantial linearization.
Calibrating to several temperature points may be necessary. For better
precision, more expensive and tighter tolerance thermistor arrays are available
to help overcome this nonlinearity but are generally less sensitive than single
thermistors.

Figure 2: Thermistor resistance
versus temperature

Because of the increased complexity and cost of multiple trip-point systems,
low-cost thermistors are typical only in applications with minimal functionality
requirements, including toasters, coffee makers, refrigerators and hair dryers.
Thermistors also suffer from self-heating, usually at higher temperatures where
their resistances are lower. As with RTDs, there are no fundamental reasons why
you cannot use thermistors with low supply voltages – keeping in mind that the
full-scale output is lower, which translates directly to less system sensitivity
based on the analog-to-digital converter (ADC) characteristics. Low-power
applications also require increased circuit complexity to be sensitive to
noise-induced errors. Thermistors operate in the -100°C to +500°C temperature
range, although most are rated for maximum operating temperatures between +100°C
and +150°C.

Thermocouples

A thermocouple consists of a junction of two wires made of different
materials. A Type J thermocouple, for example, is made from iron and constantan.
As shown in Figure 3, junction 1 is located at the temperature to be measured,
while junctions 2 and 3 are kept at a different temperature measured with an LM35
analog temperature sensor. The output voltage is approximately proportional to
the difference in these two temperature values.

Figure 3: Using the LM35
for thermocouple cold-junction compensation

Because a thermocouple’s sensitivity is rather small (on the order of tens of
microvolts per degrees Celsius), you would need a low-offset amplifier to
produce a usable output voltage. Nonlinearities in the temperature-to-voltage
transfer function over a thermocouple’s operating range often necessitate
compensation circuits or lookup tables, as with RTDs and thermocouples. However,
despite these drawbacks, thermocouples are very popular, particularly for ovens,
water heaters, kilns, test equipment, and other industrial processes, because of
their low thermal mass and wide operating temperature range, extending beyond
2300°C.

IC sensors

IC sensors can operate within a temperature range from -55°C to +150°C – a
select few can go as high as +200°C. There are various types of integrated IC
sensors, but the four most common are analog output devices, digital interface
devices, remote temperature sensors, and those with thermostat functionality
(temperature switches). Analog output devices (usually a voltage, but some with
current outputs) are most like passive solutions in their need for an ADC to
digitize the output signal. Digital interface devices most commonly use a
two-wire interface (I2C or PMBus) and have an ADC built in.

Aside from also including a local temperature sensor, remote temperature
sensors have one or more inputs to monitor a remote diode temperature, which are
most often located in a highly integrated digital IC (for example, a processor
or field-programmable gate array [FPGA]). Thermostats provide a simple alert
when a temperature threshold is reached.

There are many benefits to using IC sensors, including low power consumption,
package offerings (some as small as 0.8 mm x 0.8 mm) and low device cost in
certain applications. Furthermore, since IC sensors are calibrated during
production testing, there is no need to calibrate further. They are often used
in fitness tracking, wearables, computing systems, data loggers and automotive
applications.

Savvy board designers will make use
of the most appropriate solution depending on the end product requirements.
Table 1 shows the relative advantages/disadvantages of each type of temperature
sensor.

Table 1: The relative
advantages and disadvantages of RTDs, thermistors, thermocouples and IC
sensors

Stay tuned for more temperature basics blog posts from me and my colleagues
in the coming months. Please comment below and let us know if there are any
topics you would like us to cover.

More resources

Check out TI’s sensing
products.

Search TI’s temperature
sensor portfolio.

Search for solutions, get answers and solve problems with fellow engineers
in the temperature-sensing forum in the TI
E2E™ Community.

Read the application note, “Semiconductor
Temperature Sensors Challenge Precision RTDs and Thermistors in Building
Automation.”

Read TI’s blog post, “Ditch
the NTC thermistor: use an analog temp sensor.”

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