Temperature Sensing

Description

There is a very wide range of temperature sensing and control applications in the world today and hence many design alternatives. This solution offers in-depth design information and circuits for building thermal sensing signal chains using the most popular thermal sensors.

Usually the first step in designing a thermal sensing and control system is to determine the temperature range that must be sensed as well as the operating environment. The next step is selecting a thermal sensor. There are four main type of thermal sensors: silicon, thermistor, RTD, and thermocouple. Maxim provides either complete signal chain solutions or integrated ICs that can take the thermal transducer signal, process it, and provide either an analog or digital communication path back to the control device.

Click the design considerations tab to gain an understanding of the key parameters and circuitry needed to build a temperature sensing function. Click the "circuits" or "block diagrams" tab to view reference designs and products suggested for use in various temperature sensing applications.


The first step in designing a temperature sensor circuit is to select the temperature transducer that you are going to use. To do this, you need to know the medium you are measuring (air, water, liquid, solid) and the temperature range that you are measuring. Then you need to know the accuracy of the measurements that you need to make over the measurement range.

Popular thermal transducers include:

  • Thermocouple (range of -180°C to +1300°C)
  • RTD (range -200°C to +900°C)
  • Thermistor (range: -50°C to +150°C)
  • Silicon Sensor (range -20°C to +100°C)

While the range of the sensor that you select must meet that of your application, additional selection criteria generally includes mounting options and cost of both the sensor and the supporting signal chain.

After the transducer is selected, the next step is determining how to extract a usable signal from the transducer and deliver that signal to a controller. The signal extraction circuitry is called the signal chain. For each transducer there are signal chain alternatives, including single chip solutions. Factors in selecting which signal chain to use include accuracy, flexibility, ease of design, and cost.

This page presents some essential design considerations for different popular temperature transducer types.

Thermocouples


Thermocouples are made by joining two wires of dissimilar metals. The point of contact between the wires generates a voltage that is approximately proportional to temperature. Characteristics include wide temperature range (up to +1800°C), low-cost (depending on package), very low output voltage (about 40µV per °C for a K type), reasonable linearity, and moderately complex signal conditioning. Thermocouples require a 2nd temperature sensor (cold-junction compensation) that serves as a temperature reference and signal conditioning requires a look-up table or algorithm correction.

This table shows the output voltage vs. temperature for popular thermocouple types:

Type Temperature Range (°C) Nominal Sensitivity ( µV/°C)
K −180 to +1300 41
J −180 to +800 55
N −270 to +1300 39

The curve below (Figure 1) shows voltage output over temperature range. The curve is reasonably linear, although it clearly has significant deviations from absolute linearity.

Figure 1. Type K thermocouple output voltage vs. temperature.
Figure 1. Type K thermocouple output voltage vs. temperature.

The diagram below shows the deviation from a straight-line approximation, assuming a linear output from 0°C to +1000°C for an average sensitivity of 41.28µV/°C. To improve accuracy, linearity correction can be done by calculating the actual value or by using a lookup table.

Figure 2. Type K thermocouple deviation from a straight-line approximation.
Figure 2. Type K thermocouple deviation from a straight-line approximation.

Measuring temperature with a thermocouple can be challenging if the temperature range is narrow because the output of the thermocouple is so low. It is also complicated because additional thermocouples are created at the point where the thermocouple wires make contact with the copper wires (or traces) that connect to the signal conditioning circuitry. This point is called the cold junction (see Figure 3).

Figure 3. Simple thermocouple circuit.
Figure 3. Simple thermocouple circuit.

A complete thermocouple-to-digital circuit is shown in Figure 4. A precision op amp and precision resistors provide gain to the thermocouple output signal. A temperature sensor at the cold junction location is monitored to correct for cold junction temperature, and an ADC provides output data at the resolution required. In general, calibration is necessary to correct for amplifier offset voltage, as well as resistor, temperature sensor, and voltage reference errors, and linearization must be performed to correct for the effect of the thermocouple's nonlinear temperature-voltage relationship.

Figure 4. Example of a thermocouple signal-conditioning circuit.
Figure 4. Example of a thermocouple signal-conditioning circuit.

Maxim manufactures a dedicated single chip thermocouple interface that performs the signal conditioning functions for a variety of thermocouple types, thus simplifying the design task and significantly reducing the number of components required to amplify, cold-junction compensate, and digitize the thermocouple's output. The IC is listed under the circuits tab.

Maxim Thermocouple Solutions

Maxim offers both single chip and discrete signal chain alternatives for use with thermocouple sensors. Maxim's single chip Thermocouple-to-Digital interface IC is the MAX31855.

Click on the circuits library tab to view IC solutions and the block diagrams tab for further circuit examples. Additional design information is available in the application notes listed under "Tech Docs."

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Resistance temperature detectors - RTDs


RTDs are essentially resistors whose resistance varies with temperature. Characteristics include a wide temperature range (up to 800°C), excellent accuracy and repeatability, reasonable linearity, and the need for signal conditioning. Signal conditioning for an RTD usually consists of a precision current source and a high-resolution ADC. While RTD are fairly standardized their cost can be high depending on the base material. Platinum is the most common RTD material and Platinum RTDs, referred to as PT-RTDs are the most accurate, other RTD materials include Nickel, Copper, and Tungsten (rare). RTDs are available in probes, in surface-mount packages, and with bare leads.

One factor in determining the useful range of the RTD is the RTD package. The RTD can be made by depositing platinum onto a ceramic substrate or using a platinum wire element housed in a package. The difference in expansion rate of the substrate or package versus the platinum element can cause additional error.

For PT-RTDs, the most common values for nominal resistance at 0°C are 100Ω (PT100), 500Ω(PT500) and 1kΩ (PT1000), although other values are available. The average slope between 0°C and +100°C is called alpha (α). This value depends on the impurities and their concentrations in the platinum. The two most widely used values for alpha are 0.00385 and 0.00392, corresponding to the IEC 751 (PT100) and SAMA standards.

The resistance vs. temperature curve is reasonably linear, but has some curvature, as described by the Callendar-Van Dusen equation:

R(T) = R0(1 + aT + bT2 + c(T - 100)T3)

More information about this equation can be found in the Maxim Thermal Handbook.

The diagram below, Figure 5, shows the curve of resistance vs. temperature for a PT100 RTD along with a straight-line approximation using α. Note that the straight-line approximation is accurate to better than ±0.4°C from -20°C to +120°C.


Figure 5. PT100 RTD resistance vs. temperature. Also shown is the straight-line approximation for 0°C to +100°C.

Figure 6, below, shows the error (in degrees) between the actual resistance and the value calculated from the straight-line approximation:


Figure 6. PT100 nonlinearity compared to linear approximation based on the slope from 0°C to +100°C.

Signal conditioning for a simple 2-wire RTD usually consists of a precision resistor (reference resistor) connected in series with the RTD. A current source that forces current through the RTD and the precision reference resistor, and across the inputs of a high-resolution ADC. The voltage across the reference resistor is the reference voltage for the ADC. The ADC's conversion result is simply the ratio of the RTD's resistance to the reference resistance. An example of a simple RTD signal-conditioning circuit is shown in Figure 7.

Several variations are common. The current source may be integrated into the ADC, or the current source may be eliminated and a voltage source may be used to provide bias to the RTD-RREF divider. This approach is not as common as providing a current supply because the voltage supply provides accurate results only when the wires connecting the RTD to circuit have very low resistance.

Figure 7. Simplified RTD signal-conditioning circuit.
Figure 7. Simplified RTD signal-conditioning circuit.

3-Wire or 4-Wire RTD Interface

When the RTD's cable resistance is significant (greater than a few mΩ for a PT100), a 3-wire or 4-wire RTD will generally be used. Four wires allow force and sense connections to the RTD to eliminate the effect of wire resistance. Three wires provide a compromise solution that partially cancels the effect of cable resistance. Linearization is generally done using a lookup table, although external linear circuits can provide good linearization over a limited temperature range.

To measure the resistance of an RTD, a small electric current (about 1 mA) must flow through the sensor to create the necessary voltage drop. The current causes the platinum element in the RTD to heat up above the temperature of the RTD's environment (also called Joule heating). The heating is proportional to the electric power (P=I2R) in the RTD and the heat transfer between the RTD sensing element and the RTD environment.

The most common standards for RTD tolerances are the American standard (ASTM E1137) Grades A and B and European standard IEC 751 Class A or B.

ASTM E1137 IEC 60751 (2008)
Grade Tolerance Class Tolerance
A ±(0.13 + 0.0017 |t|)  A (Class F0.15) ±(0.15 + 0.002 |t|) 
B ±(0.25 + 0.0042 |t|)  B (Class F0.3) ±(0.3 + 0.005 |t|)

Where |t| is the absolute value of temperature in °C

Maxim RTD Solutions

Maxim offers both single chip and discrete signal chain alternatives for use with RTD sensors. Maxim's single chip RTC-to-Digital interface is the MAX31865.

Click on the circuits library tab to view IC solutions and the block diagrams tab for circuit examples. Additional design information is available in the application notes listed under "Tech Docs."

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Thermistor


Thermistors are temperature-dependent resistors, usually made from conductive materials such as metal-oxide ceramics or polymers. The most common thermistors used for temperature sensing have a negative temperature coefficient (NTC) of resistance. Thermistors are available in probes, in surface-mount packages, with bare leads, and in a variety of specialized packages.

Characteristics include moderate temperature range (generally up to +150°C, though some are capable of much higher temperatures), low-to-moderate cost (depending on accuracy), poor but repeatable linearity. The linearity of a thermistor varies significantly over temperature. Over a range of 0° to 70°C thermistor non-linearity can be ±2°C to ±2.5°C while over a range 10° to 40°C typical non-linearity can be ±0.2°C.

A simple, common approach to using a thermistor is to use a voltage divider as shown in Figure 8, where a thermistor and fixed-value resistor form a voltage divider whose output is digitized by an analog-to-digital converter (ADC).

Figure 8. This basic circuit shows how a thermistor can interface to an ADC. Resistor R1 and the thermistor form a voltage divider with a temperature-dependent output voltage.
Figure 8. This basic circuit shows how a thermistor can interface to an ADC. Resistor R1 and the thermistor form a voltage divider with a temperature-dependent output voltage.

NTC thermistors have a large negative temperature coefficient over wide temperature ranges. The relationship between resistance and temperature for a common NTC is shown in Figure 9. This is an issue for both linear and logarithmic correction over wide temperature ranges.


Figure 9. Resistance vs. temperature curves for a standard NTC. Nominal resistance is 10kΩ at +25°C. Note the nonlinearity and large relative temperature coefficient of curve (a). Curve (b) is based on a logarithmic scale and also exhibits significant nonlinearity.

An NTC's nonlinearity over a wide temperature range can affect the choice of the ADC selected to digitize the temperature signal. Since the slope of the curves in Figure 9 decreases significantly at temperature extremes, the effective temperature resolution of any ADC used with the NTC thermistor is limited at those extremes and this often requires the use of a higher resolution ADC.

Combining an NTC with a fixed resistor in a voltage-divider circuit like the one in Figure 8 provides some linearization, as shown in Figure 10. By selecting an appropriate value for the fixed resistor, the temperature range for which the curve is most linear can be shifted to meet the needs of the application.


Figure 10. Making an NTC voltage-divider, as in Figure 9, helps to linearize the NTC's resistance curve over a limited temperature range. The voltages on the NTC and the external resistor, R1, are shown as a function of temperature. Note that the voltage is roughly linear from 0°C to +70°C.

For wide temperature range applications a common approach is to use the Steinhart–Hart equation. This provides a third order approximation. The error in the Steinhart–Hart equation is generally less than 0.02⁰C over a measurement range of 200°C range.

More information about Steinhart-Hart equation can be found in the Maxim Thermal Handbook.

Maxim Thermistor Solutions

Maxim manufactures a few different single chip thermistor based digital output ICs. While the MAX31865 was designed for use with RTDs, it is also a very good choice for use with a thermistor.

Click on the circuits library tab to view IC solutions and the block diagrams tab for circuit examples. Additional design information is available in the application notes listed under "Tech Docs."

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Silicon


Silicon temperature sensors are available with analog or digital outputs. While the range of a silicon sensor is limited, they are easy to use and many have additional features like thermostat functions.

Analog Temp Sensors

An analog temperature sensor is useful in applications where the output needs to be sent through a current loop to a monitoring device. Digital outputs can also be converted in this case, but then the signal goes through two extra conversion steps.

Analog temperature sensor ICs use the thermal characteristics of bipolar transistors to develop an output voltage or, in some cases, current, that is proportional to temperature.

The simplest analog temperature sensors have just three active connections: ground; power supply voltage input; and output. Other analog sensors with enhanced features may have additional inputs or outputs such as a comparator or voltage reference output.

Figure 11 shows a curve of output voltage vs. temperature for a typical analog temperature sensor, the MAX6605. Figure 12 shows the deviation from a straight line for this sensor. From 0°C to +85°C, the linearity is within about ±0.2°C, which is quite good compared to thermistors, RTDs, and thermocouples.

Figure 11. Output voltage vs. temperature for the MAX6605 analog temperature-sensor IC.
Figure 11. Output voltage vs. temperature for the MAX6605 analog temperature-sensor IC.


Figure12. The MAX6605 output voltage deviation from a straight line. Linearity from 0°C to +85°C is approximately ±0.2°C.

Analog temperature sensors can have excellent accuracy. For example, the DS600 has a guaranteed accuracy of ±0.5°C from -20°C to +100°C. Other analog sensors are available with larger error tolerances, but many of these have very low operating current (on the order of 15µA, max) and are available in small packages (e.g., SC70).

Digital Temperature Sensors

Integrating an analog temperature sensor with an ADC is an easy way to create a temperature sensor with a direct digital interface. Such a device is normally called a digital temperature sensor, or a local digital temperature sensor. "Local" refers to the fact that the sensor measures its own temperature, as opposed to a remote sensor that measures the temperature of an external IC or discrete transistor.

Figure 13 shows block diagrams for two digital temperature sensors. Figure 13a illustrates a sensor that simply measures temperature and clocks the resulting data out through a 3-wire digital interface. Figure 13b shows a sensor that includes several additional features, such as over-/under temperature outputs, registers to set trip thresholds for these outputs, and EEPROM.


Figure 13. Block diagrams of local digital temperature sensors. (a) Simple sensor with serial digital output. (b) Sensor with additional functions, such as over-/under temperature alarm outputs and user EEPROM.

One advantage of using a digital temperature sensor is that all of the errors involved in digitizing the temperature value are included within the sensor's accuracy specifications. In contrast, an analog temperature sensor's specified error must be added to that of any ADC, amplifier, voltage reference, or other component that is used with the sensor. A good example of a very high-performance digital temperature sensor is the MAX31725, which achieves ±0.5°C accuracy across a temperature range of -40°C to +105°C. The MAX31725 can be used over a range of -55°C to +125°C temperature range and provides a maximum temperature error of just ±0.7°C with a 16-bit (0.00390625°C) resolution.

Most digital temperature sensors include one or more outputs that indicate that the measured temperature has gone beyond a preset (usually software-programmable) limit. The output may behave like a comparator output, with one state when temperature is above the threshold and the other state when temperature is below the threshold. Another common implementation is for the output to behave as an interrupt that is reset only in response to an action by the master.

Digital temperature sensors are available with a wide variety of digital interfaces including I2C, SMBus™, SPI™, 1-Wire®, and PWM.

Maxim Analog and Digital Silicon-Based Temperature Solutions

Maxim offers a variety of silicon-based temperature sensors with analog or digital output.

Click on the circuits library tab to view IC solutions and the block diagrams tab for circuit examples. Additional design information is available in the application notes listed under "Tech Docs."

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Integrated and Accurate RTD and Thermocouple Temperature Transmitter


RTD to Digital Thermocouple to Digital Reference LDO DAC Op Amp IO-Link RS-485 DC-DC

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Portable Temperature Calibrator


Drive Op Amp Reference Audio Amp RS-232 LCD Display USB Supervisory Temp Reference Battery Charger Current Sensing LDO Step-Down DC-DC Converter Isolated Power Over V/I Protection RTD AFE TC AFE

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Type ID PDF Title
Tutorial 4679 Thermal Management Handbook