Temperature sensors are improving, but select carefully

From thermocouples and thermistors to resistance-temperature-detectors (RTDs), temperature sensors are varied and ubiquitous, but it would be wise not to take them for granted. Each type comes with its own set of inherent pros and cons in terms of cost, reliability, linearity, and ease of use. In this feature we will take you through some classics, while also updating you on the state of the art, how to make best of them, and a good example or two of temperature sensor and silicon integration. 

Temperature sensors really are everywhere: used in automotive, infrastructure, industrial, military/aerospace, consumer electronics, medical, transportation, power, process control, petro-chemical, and geo-physical, agriculture, and communications applications. When used in combination with other sensors like strain and pressure sensors, they’re making the applications they serve more intelligent, safer, and more reliable. In many systems, temperature monitoring and control is fundamental. 

By far, the biggest challenge designers face when using temperature sensors is how and where the sensor is placed with respect to the object or environment being measured (the measurand). Even the type of package and how it is mounted can make a difference between satisfactory and unsatisfactory measurements. With signal levels so low, losses in the connecting wires are but one of the issues to be overcome, but more on that in upcoming features.

In the meantime, one popular type of temperature sensor is the thermocouple, and for good reason. No other temperature sensor can rival it in terms of its wide-ranging temperature sensing ability of minus hundreds of °C to nearly 2000°C. It is also rugged and can withstand harsh environments.

But it’s also highly non-linear and requires the use of significant linearization algorithms. It also produces very small output voltages of tens of µV/°C, requiring the use of analog amplification, and is susceptible to noise which means signal conditioning is required. And it is fairly accurate provided that accurate correct cold-junction compensation (CJC) and signal conditioning is provided. Experts agree that the signal-conditioning aspect of a thermocouple is a major challenge for temperature sensor users. 

Fortunately, there are many inexpensive monolithic ICs on the market that help, like the Analog Devices ADT7420/ADT7320 (I2C/SPI outputs). It allows thermocouple cold-junction compensation with accuracy within ±0.25°C (Figure 1). Together with Analog Devices’ AD849x series of precision amplifiers, it provides convenient analog CJC for modern thermocouple sensor types.

Figure 1 Analog Devices’ ADT7420/7320 16-bit digital temperature sensor with I2C/SPI outputs allows thermocouple cold-junction compensation with accuracy within ±0.25°C. (Source: Analog Devices)

The ADT7320/7420 16-bit sensors also have the added advantage of not requiring calibration. Recommended for new designs by ADI, start with the EVAL-ADT7420 evaluation board. 

Thermistors: Low cost, but keep one eye on current

Another popular temperature sensor type is the thermistor, not least because of it’s very low cost (a few cents in large quantities). This thin-film sensor is quite small (a bead of about 4 mm in diameter with two leads) and it suits applications like over- and under-voltage shutdown instrumentation. It is highly sensitive and accurate, rugged, and flexible enough to be fitted into a variety of packages.

But it does have one problem. It is a self-heating device so proper care must be taken to limit the current being sensed to a low-enough value to minimize this challenge. It is also susceptible to moisture failure, though modern glass-encapsulated thermistors can be purchased to handle this problem, adding extra cost, of course. And software is needed to handle a thermistor’s inherent non-linearity.

Thermistors come in positive temperature coefficient (PTC) and negative temperature coefficient (NTC) versions that increase and decrease their resistances in response to increasing and decreasing temperatures, respectively.  

RTDs: Stable and linear, but slow response

For a higher price, a resistance-temperature detector (RTD) offers the best linearity and stability over a reasonably wide temperature range of about −250°C to 850°C, which is second only to a thermocouple. An RTD consists of a deposited film of copper or platinum. Its resistance increases with increasing temperatures in a known and repeatable manner, with excellent accuracy levels. RTDs like the F2020 series from Omega Engineering offer designers a flat 2×2 mm factor (Figure 2).

Figure 2 An RTD like this Omega Engineering F2020 series is a good choice for linear and stable temperature measurements with excellent accuracy. (Source: Omega Engineering)

It is made on OmegaFilm platinum material and comes in 100-Ω, 500-Ω, and 1-k Ω versions, can operate over a −50°C to 500°C range, and comes with platinum-coated nickel wires.

RTD users beware! An RTD’s response time is very slow (in the few seconds range). It is also highly susceptible to lead resistance effects thus requiring lead-wire compensation, limiting its use in 3-wire and 4-wire configurations. (2-wire configurations cannot compensate for lead-wire resistances).

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