Optimizing RTD temperature sensing systems: Challenges

Article By : Jellenie Rodriguez and Mary McCarthy

With their accuracy and stability, RTDs play an important role in multiple end applications, but selecting and designing the best RTD temperature measurement system involves a lot of considerations.

This three-part article series discusses the history and design challenges for designing a resistance temperature detector (RTD)-based temperature measurement system. It also covers RTD selection and configuration trade-offs. Finally, it details RTD system optimization and evaluation.

Temperature measurement plays an important role in many different end applications such as industrial automation, instrumentation, CbM, and medical equipment. Whether monitoring environmental conditions or correcting system drift performance, high accuracy and precision are very important. There are several types of temperature sensors that can be used such as thermocouples, resistance temperature detectors (RTDs), electronic band gap sensors, and thermistors. The temperature sensor selected along with the design depends on the temperature range being measured and the accuracy required. For temperatures in the range of –200°C to +850°C, RTDs provide an excellent combination of high accuracy and good stability.

The primary challenges to making temperature measurements with high accuracy and good stability include:

• Current and voltage selection. An RTD sensor is a passive device and does not produce an electrical output on its own. Excitation current or voltage is used to measure the resistance of the sensor by passing a small electrical current through the sensor to generate a voltage.
• Whether to use a 2-wire, 3-wire, or 4-wire sensor.
• Conditioning the RTD signal.
• Adjusting the above variables so that the converter or other building blocks are used within their specification.
• Connecting multiple RTDs in a system, determining which blocks be shared among the different sensors, and the overall impact of these choices on system performance.
• Determining the expected error for the design.

RTD Overview

For an RTD, the resistance of the sensor varies as a function of temperature in a precisely defined manner. The most widely used RTDs are platinum Pt100 and Pt1000, which are available in 2-wire, 3-wire, and 4-wire configurations. Other RTD types are made from nickel and copper.

Table 1. Common RTD Types

The most common Pt100 RTDs can take two different shapes: wire wound and thin film. Each type is built to several standardized curves and tolerances. The most common standardized curve is the DIN curve. DIN stands for “Deutsches Institut für Normung,” which means “German institute for standardization.” The curve defines the resistance vs. temperature characteristics of a platinum 100 Ω sensor, the standardized tolerances, and the operating temperature range. This defines the accuracy of the RTD starting with a base resistance of 100 Ω at a temperature of 0°C. There are different standard tolerance classes for DIN RTDs. These tolerances are shown in Table 2, and they also apply to Pt1000 RTDs that are useful in low power applications.

Table 2. RTD Accuracy—Class A, Class B, 1/3 DIN

Both the RTD itself and its accuracy must be considered when selecting the RTD sensor. The temperature range varies with element type, and the accuracy denoted at calibration temperature (usually at 0°C) varies with temperature. Thus, it is important to define the temperature range being measured and take into consideration that any temperature below or above the calibration temperature will have a wider tolerance and lower accuracy.

RTDs are categorized by their nominal resistance at 0°C. A Pt100 sensor has a temperature coefficient of approximately 0.385 Ω/°C and a Pt1000 has a temperature coefficient that is a factor of 10 greater than the Pt100. Many system designers use these coefficients to get an approximate resistance to temperature translation, but the Callendar-Van Dusen equations provide a more accurate translation.

The equation for temperature t ≤ 0°C is

The equation for temperature t ≥ 0°C is

where:

t is the RTD temperature (°C)

R_RTD(t) is the RTD resistance at temperature (t)

R₀ is the RTD resistance at 0°C (in this case, R₀ = 100 Ω) A = 3.9083 × 10⁻³

B = −5.775 × 10⁻⁷

C = −4.183 × 10⁻¹²

RTD Wiring Configurations

Another sensor parameter that needs to be considered when selecting an RTD is its wiring configuration, which will affect system accuracy. There are three different RTD wiring configurations available in the market wherein each configuration has advantages and disadvantages over one another and may require different techniques to reduce the measurement error.

A 2-wire configuration is the simplest but the least accurate configuration due to errors in lead-wire resistance and its variation with temperature contributing a significant measurement error. Thus, this configuration is only useful in applications where lead wires are short or when using a high resistance sensor (for example, Pt1000), both of which minimize lead resistance effects on the accuracy.

3-wire is the most used configuration because of the advantage of using three pins, which are useful in designs where the connector size is minimized (three connection terminals required vs. the 4-wire terminal for a 4-wire RTD). 3-wire also has significant accuracy improvement over the 2-wire configuration. The lead-wire resistance error in 3-wire can be compensated using different calibration techniques that will be later covered in this article.

4-wire is the most expensive but the most accurate configuration. In this configuration, the errors due to lead-wire resistance, along with temperature variation effects, are removed. Therefore, a 4-wire configuration results in the best performance.

RTD Configuration Circuit

A high precision and accurate RTD sensor measurement requires precise signal conditioning, analog-to-digital conversion, linearization, and calibration. The typical design of an RTD measurement system consists of the different stages as shown in Figure 2. Although the signal chain looks simple and straightforward, there are several complex factors involved and designers must consider complex component selection, connection diagram, error analysis, and other analog signal conditioning challenges that impact overall system board size and the cost of the bill of materials (BOM) due to the higher number of contributing blocks. On the brighter side, there are plenty of integrated solutions available in ADI’s portfolio. This complete system solution helps designers to simplify their designs while reducing the board size, time to market, and the cost of the overall RTD measurement system.

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Figure 1: RTD wiring configurations. (Source: Analog Devices)

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Figure 2: Typical RTD measurement signal chain block. (Source: Analog Devices)

The three RTD wiring configurations have different wiring techniques needed to interface or connect an RTD to an ADC, along with the other external components, and requirements from the ADC, such as excitation current and a flexible mux. This section covers a deeper understanding and focus on each RTD configuration circuit design and considerations.

Sigma-Delta ADCs

Sigma-delta (Σ-Δ) ADCs offer multiple benefits when designing RTD systems. Firstly, as sigma-delta ADCs oversample the analog input, external filtering is minimized, with a simple RC filter being the only requirement. They offer flexibility in terms of choice of filter type and choice of output data rate. The inbuilt digital filtering can be used to reject any interference from the mains power supply in mains operated designs. 24-bit, high resolution ADCs such as the AD7124-4/AD7124-8 have a peak-to-peak resolution of 21.7 bits maximum. Other benefits are

• Wide common-mode range for the analog inputs
• Wide common-mode range for the reference inputs
• Ability to support ratiometric configurations
• Buffered reference and analog inputs
• Some sigma-delta ADCs are highly integrated and include
• A programmable gain amplifier (PGA)
• Excitation currents
• Reference/analog input buffers
• Calibration functions

They simplify the RTD design significantly along with reducing the BOM, system cost, board space, and time to market.

For this article, the AD7124-4/AD7124-8 are used as the ADC. These are low noise, low current precision ADCs with an integrated PGA, excitation currents, analog input, and reference buffers.

Ratiometric Measurement

A ratiometric configuration is a suitable and cost-effective solution for systems that use resistive sensors such as RTDs or thermistors. With a ratiometric approach, the reference and sensor voltages are derived from the same excitation source. Therefore, the excitation source does not need to be accurate. Figure 3 shows an example of a ratiometric configuration in a 4-wire RTD application. A constant excitation current supplies the RTD and a precision resistor, R_REF, with the voltage generated across R_REF being the reference voltage for the RTD measurement. Any variation of the excitation current does not affect the accuracy of the measurement. Therefore, using a ratiometric approach allows a noisier, less stable excitation current to be used. An excitation current is preferred over voltage excitation due to its better noise immunity. The major factors to consider when selecting an excitation source value are discussed later in this article.

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Figure 3. 4-wire RTD ratiometric measurement. (Source: Analog Devices)

Shared IOUT/AIN Pin

Many RTD system designers use sigma-delta ADCs with integrated mux and excitation currents that allow multiple channel measurements and flexible routing of the excitation currents to each sensor. An ADC such as the AD7124 allows a single pin to operate simultaneously as an excitation current and an analog input pin (see Figure 4). Sharing pins between IOUT and AIN will only require two pins per 3-wire RTD sensor, which increases the channel count. However, in this configuration, a large value of the resistor R in the antialiasing or electromagnetic interference (EMI) filtering can add errors to the RTD resistance value as R is in series with the RTD—thus, limited R values can be used. That’s why it is usually recommended to have a dedicated pin for each excitation current source to avoid possible errors across RTD measurements.

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Figure 4. 3-wire RTD with a shared IOUT/AIN pin. (Source: Analog Devices)

In part 1, we covered temperature measurement challenges, RTD types, different configurations, and the RTD configuration circuit. In part 2, we’ll cover the three different RTD configurations: 2-wire, 3-wire, and 4-wire.

This article was originally published on Embedded.

Jellenie Rodriguez is an applications engineer at Analog Devices within the Precision Converter Technology Group. Her focus is on precision sigma-delta ADCs for DC measurements. She joined ADI in 2012 and graduated from San Sebastian College-Recoletos de Cavite with a bachelor’s degree in electronics engineering in 2011. She can be reached at jellenie.rodriguez@analog.com.

Mary McCarthy is an applications engineer at Analog Devices. She joined ADI in 1991 and works in the Linear and Precision Technology Applications Group in Cork, Ireland, focusing on precision sigma-delta converters. Mary graduated with a bachelor’s degree in electronic and electrical engineering from University College Cork in 1991. She can be reached at mary.mccarthy@analog.com.

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