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Temperature detection is widely used in our daily lives and industrial settings, and the accuracy of temperature measurement circuits has become increasingly important. How can we improve the accuracy of these circuits? Taking the thermal resistor temperature measurement scheme as an example, this article will start with the selection parameters for thermal resistors to briefly explain the approaches to improving the accuracy of temperature measurement. Platinum resistance thermometers exhibit good long-term stability and accuracy, making them common industrial temperature sensing elements. In recent years, thin-film printing manufacturing processes have reduced the amount of precious metal platinum used, leading to a significant drop in the cost of platinum thermistors, which has enabled their widespread adoption. When using platinum resistance thermometers in conjunction with subsequent circuits, it is important to pay attention to three basic parameters: their nominal resistance, temperature coefficient, and accuracy grade. These parameters help us determine the appropriate choice of platinum resistance thermometer. Understanding the characteristics of temperature-to-resistance conversion, the measurement current, and wiring methods can assist us in minimizing additional circuit errors and establishing an accurate temperature measurement system. Nominal resistance: The nominal resistance is the resistance value of a platinum thermistor at 0°C, the freezing point. PT100 with a nominal resistance of 100Ω is the most commonly used; there are also PT200, PT500, and PT1000 with nominal resistances of 200Ω, 500Ω, and 1000Ω respectively. Temperature coefficient: The temperature coefficient TCR refers to the average change in the resistance value of a platinum thermistor per unit of temperature between the freezing and boiling points of water. Different organizations use different temperature coefficients as their standards; the temperature coefficient adopted in European standard IEC60751 and Chinese standard GB/T30121 is 0.003851, while that adopted in American standard ASTM E1137 is 0.003902. At present, 0.003851 is the industry standard recognized in China and most other countries. The calculation process for the temperature coefficient is as follows, taking PT100 as an example. TCR = (R100 – R0) / (R0 × 100). The resistance value at a boiling point of 100°C is R100 = 138.51 Ω, while the resistance value at a freezing point of 0°C is R0 = 100 Ω. By dividing the difference of 38.51 by the nominal resistance and then by 100, the average temperature coefficient is obtained. The accuracy class for platinum resistance thermometers is specified in IEC60751, along with the allowable errors. Taking a Class A platinum resistance thermometer as an example, the maximum temperature error consists of two components: a fixed error of 0.15°C caused by the deviation from the nominal resistance value at 0°C, plus an error of 0.002×|T| introduced by the drift of the temperature coefficient. Here, T represents the actual temperature measurement range; as long as T does not exceed the application temperature range of -30~+300°C specified in the accuracy class table, the platinum resistance thermometer will remain within the allowable error limits of that accuracy class. When the measured temperature is 100°C, the total error of the Class A platinum resistance thermometer is 0.15 + 0.002×100 = 0.35°C. When making a selection, the nominal resistance of the platinum thermistor, the standards for its temperature coefficient, the accuracy grade, and the operating temperature range are the criteria we use for our decision. Temperature-resistance conversion characteristics: The temperature-resistance relationship of platinum thermoresistors is described by the following formula, with separate considerations for temperatures below 0°C and above 0°C. When T≤0℃: RT=R0? (1+A?T+B?T2+C? (T-100℃) ?T3) When T≥0℃: RT=R0? (1+A?T+B?T2) Where RT is the resistance value at temperature T, and R0 is the resistance value at 0℃ ； A, B, and C are three constants specified in IEC60751, with values of 3.9083×10-3 °C-1, -5.775×10-7 °C-2, and -4.183×10-12 °C-4 respectively. The measured temperature T can be determined by directly substituting the resistance value RT into the formula, but this requires solving a cubic equation, which makes the calculation complex. To simplify the calculations, the formula is used to generate the temperature-resistance curve of PT100 in the range of -200 to +850°C, as shown in the figure below. The resistance value of PT100 varies within the range of 18 to 400Ω, exhibiting an approximately linear relationship between temperature and resistance. If a two-point linear calibration is performed using the endpoints of -200°C and +850°C to simplify the calculations, the temperature-resistance value curve within that temperature range is shown in the figure below. At this point, the maximum nonlinear error exceeds 16Ω, resulting in a relatively large error. Generating a temperature-resistance value table based on formulas, followed by performing small-scale linear interpolation in that lookup table, is a method that is both simple to compute and capable of achieving accurate approximation. IEC60751 includes a lookup table for temperature resistance values at intervals of 1°C. To measure current, platinum thermoresistors are almost always driven by direct current for measurement. The measuring current inevitably generates heat within the resistor, introducing self-heating errors. The manual for platinum thermistors specifies two parameters: the measurement current and the self-heating coefficient. The typical measurement current I ranges from 0.3 to 1 mA, while the self-heating coefficient S is around 0.015℃/mW. The temperature error introduced by the measuring current can be calculated based on the self-heating coefficient, using the following formula. ΔT = P×S = (I²×R) × S. For example, with a current of 1 mA and a maximum resistance of 400Ω for a PT100 sensor, the self-heating temperature generated is approximately 0.01°C; in this case, the error is practically negligible. When the self-heating coefficient of the platinum resistor has no impact, the measurement current is preferably set to its maximum value; when the current is too low, the amplitude of the output voltage decreases and the signal-to-noise ratio drops. 1mA is a commonly used measurement current value. Wiring methods: The output leads of platinum resistance thermometers can be connected in a two-wire, three-wire, or four-wire configuration; however, the error introduced by the resistance of the two-wire connection cannot be eliminated ； Four-wire configuration eliminates lead resistance errors, but the maximum number of leads is limited ； The three-wire system relies on three base leads; under identical physical dimensions, the resistance values of these leads are equal. By measuring the resistance values twice and performing calculations, lead-related errors can be eliminated, making it the most commonly used method. In summary, the accuracy of a temperature measurement circuit depends not only on the proper selection of the thermal resistor but also on the optimization of the subsequent hardware design and software algorithms. Zhiyuan Electronics offers the PTS02 interface module for PT100 resistors, which features a three-wire interface, a highly stable measurement circuit including an internal excitation current source, a 24-bit ADC, an algorithm for linearizing resistance-temperature values, and 2500V electrical isolation. By connecting it to a platinum thermal resistor, it is possible to read the temperature value via an IIC digital interface. Shanghai No. 3 Factory of Automatic Instruments: http://www.zdhybsc.cn/