Temperature measurement is a very common type of measurement in any system design. Many end devices have several ways to measure temperature as key data. RTDs, thermistors, and thermocouples represent analog components in temperature measurement. In different system designs, the appropriate temperature sensing method needs to be selected according to the required accuracy and the temperature range to be measured.
Thermistors are a class of temperature sensing that we have often focused on previously. As a simple, small-sized solution, it has an extremely fast response time, with around 1 second at the NTC chip level and up to less than 15 seconds or even less than 10 seconds at the sensor level. However, its non-linearity is poor and not durable enough compared to other temperature measurements, and the thermistor's self-heating will impact the accuracy. The linear PTC thermistor is not counted here. Although the linear PTC solves the various shortcomings of NTC in temperature measurement and is a good choice for precision temperature measurement, its application is not wide enough, and the cost is high.
Digital temperature sensing is popular in new application scenarios and has outstanding performance in all aspects, but the inherent disadvantage of a limited temperature range is unavoidable. The same is true for thermocouples, which are a highly accurate measurement solution but have the disadvantage of requiring cold-end compensation to provide error correction.
The RTD we are looking at here is a fairly durable and accurate enough option in applications with a wide temperature measurement range (-200°C to 850°C). Over the entire temperature range, the RTD's response can exhibit an almost linear characteristic, and the sensitivity can be as high as several hundred µV/°C. Whether you choose a 3-wire or 4-wire solution, it is very reliable for precision temperature measurement.
The 2-wire configuration is the simplest, but due to the inherent inaccuracy of the sensor lead resistance and the inability to compensate directly, the 2-wire solution is not used in precision temperature measurement. The most common configuration for RTDs in precision temperature measurement is a 3-wire configuration with a compensation loop to eliminate the effect of lead resistance during measurement. The measurement device first measures the total resistance of the sensor and the connected leads, then measures the compensation loop resistance to determine the actual net resistance. 4-wire RTDs are more accurate and eliminate the effect of lead resistance, making this configuration more accurate for temperature measurement. Still, many industrial controllers/measurement devices cannot achieve true 4-wire measurements and are not inexpensive.
RTD solution for high accuracy and stability of temperature measurement in a wide temperature range still has several challenges. The first is the choice of current and voltage. RTD is a passive device. To generate the measured voltage, the excitation current is required. Generally speaking, using a ratiometric configuration, the reference voltage and sensor voltage are obtained from the same excitation source, and the excitation source requirements can be less precise. Any variation in the excitation current will not affect the measurement accuracy, even if the excitation current is noisy or unstable. Generally speaking, higher excitation currents are better. Still, it is necessary to ensure that the high current resistor power consumption is too high or self-heating affects the measurement results, which requires a trade-off between current value and performance.
Stability also has several challenges. Although the RTD temperature measurement front-end can withstand a certain ESD level, the precision temperature measurement scenarios are often the more complex electromagnetic environments. For example, the coupling on the RTD sensor cable may make the stability of the entire measurement system faces EMI challenges. Especially in industrial scenarios, the longer the sensor cable, the greater this risk.
Some common protection devices are configured into the measurement system to increase the reliability of the temperature measurement system. Take the well-known TVs as an example. In the RTD temperature measurement system, the higher the breakdown voltage of the TVS to the system will bring less error. Although the addition of protection devices slightly increases the total error of the temperature measurement system, the stability is also higher. The RTD can be configured to reduce the error after adding protection devices by the excitation current.
Temperature measurement does not require high sampling speed, usually low speed, but the resolution requirements are very high. Many manufacturers provide temperature front ends that use a sigma-delta architecture to achieve this high resolution, low bandwidth ADC. Sigma-delta ADCs can oversample the analog input, further reducing external filtering. A good signal-to-noise ratio SNR is also essential to facilitate the next step in the data processing.
Some ADCs will directly integrate PGAs, excitation currents, and reference voltage sources and also enable ratiometric configuration, greatly simplifying the design of RTD temperature measurement systems.
In the precision temperature measurement, RTD wants to balance high accuracy and high stability to consider the very low noise PGA, high enough resolution ADC, and the excitation current configuration and other protection components. A good balance between these configurations is the key to a precision temperature measurement RTD sensing system.