Cover story
Temperature measurement challenges High-precision and accurate thermistor-based temperature measurement requires precise signal conditioning, analogue- to-digital conversion, linearisation and compensation; see Figure 2. Although the signal chain looks simple and straightforward, there are several complex factors that impact overall system board size, cost and performance. The AD7124-4/AD7124-8 belong to ADI’s precision ADC portfolio, providing many benefi ts in temperature systems design, since most of the building blocks required in an application are built-in. However, there are different challenges involved in designing and optimising a thermistor-based temperature measurement system, including how to select the right thermistor, how to select its current and voltage, how to condition its signal, and more. Thermistors are listed by their nominal value, which is the nominal resistance at 25°C. So, a 10kΩ thermistor has a nominal resistance of 10kΩ at 25°C. They are available with nominal or base resistance values from a few ohms to 10MΩ. Those with low nominal resistance (10kΩ or less) typically support a lower temperature range, like –50°C to +70°C; those with higher resistance support up to 300°C. The thermistor element is made from metal oxides.
Thermistors are available in bead, radial and SMD form. Bead thermistors are epoxy-coated or glass-encapsulated for extra protection. Epoxy-coated bead, radial and SMD thermistors are suitable for temperatures to 150°C; glass-coated ones are suitable for higher temperature measurements. The coating/packaging also prevents corrosion, and some thermistors will have additional housing for use in harsh environments. Bead thermistors have a faster response time vs. radial/ SMD thermistors, but are not as robust. So, the thermistor type to choose depends on the application and application environment. The long-term stability of a thermistor is dependent on its materials, packaging and construction. For example, an epoxy-coated NTC thermistor can change by 0.2°C per year, whilst a hermetically-sealed one changes by only 0.02°C per year. Thermistors have differing accuracy. Standard thermistors
typically have accuracy of 0.5-1.5°C. They have a tolerance on their nominal resistance value and on their beta value (25°C to 50°C/85°C relationship). Note that the beta value of a thermistor is dependent on the manufacturer. For example, 10kΩ NTC thermistors from different manufacturers will have different beta values. For higher accuracy systems, thermistors such as the Omega 44xxx series can be used. These have an accuracy of 0.1-0.2°C over a temperature range of 0-70°C. So, the temperature range being measured, along with the accuracy required over its range, determines whether a thermistor is suitable for the application. Note that the more accurate the Omega 44xxx series is, the higher its cost. Therefore, the thermistor to be used
Figure 4: Voltage excitation of a thermistor
Figure 5: Confi guration with a constant current source
Figure 6: Confi guration with a voltage divider circuit
www.electronicsworld.co.uk February 2024 07
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