Abstract: This paper introduces the integrated temperature sensor AD590, and presents a specific circuit for measuring thermodynamic temperature, Celsius temperature, two-point temperature difference, multi-point minimum temperature, and multi-point average temperature using AD590. An application of the two-point temperature difference measurement circuit using AD590 is illustrated using an energy-saving temperature and humidity control system as an example. Keywords: AD590; Integrated Temperature Sensor; Temperature Difference; Integrated Temperature Sensor AD590 and Its Applications Abstract: The Integrated Temperature Sensor AD590 is introduced, and the concrete circuit for measuring various temperatures is provided, such as thermodynamic temperature, Celsius temperature, temperature difference, lowest temperature, average temperature, and so on. The application circuit for measuring the temperature difference is introduced in the economic system of temperature and humidity measurement. Keywords: AD590; Integrated Temperature Sensor; Temperature difference CLC Number: TP368 TP212.11 Document Code: A Article Number: 1006-883X (2003) 03-0035-03 I. Introduction An integrated temperature sensor is essentially a semiconductor integrated circuit. It detects temperature by utilizing the following relationship between the unsaturated value VBE of the transistor's be-e junction voltage drop and the thermodynamic temperature T and the emitter current I: Where, K—Boltz constant; q—absolute value of electron charge. Integrated temperature sensors are widely used due to their advantages such as good linearity, moderate accuracy, high sensitivity, small size, and ease of use. Integrated temperature sensors have two output forms: voltage output and current output. The sensitivity of voltage output sensors is generally 10mV/K, with an output of 0 at 0℃ and 2.982V at 25℃. The sensitivity of current output sensors is generally 1mA/K. II. Introduction to AD590 The AD590 is a monolithic integrated two-terminal temperature-sensing current source manufactured by Analog Devices, Inc. Its main characteristics are as follows: 1. The current (mA) flowing through the device is equal to the thermodynamic temperature (Kelvin) of the environment in which the device is located, i.e.: Where: Ir—current flowing through the device (AD590), in mA; T—thermodynamic temperature, in K. 2. The temperature measurement range of the AD590 is -55℃ to +150℃. 3. The power supply voltage range of the AD590 is 4V to 30V. The power supply voltage can vary from 4V to 6V, and a 1mA change in current is equivalent to a 1K change in temperature. The AD590 can withstand a 44V forward voltage and a 20V reverse voltage, so the device will not be damaged even if reversed. 4. The output resistance is 710MW. 5. High accuracy. The AD590 has five ranges: I, J, K, L, and M. Among them, the M range has the highest accuracy, with a nonlinear error of ±0.3℃ in the range of -55℃ to +150℃. III. Application Circuit of AD590 1. Basic Application Circuit Figure 1(a) shows the package form of AD590, and Figure 1(b) shows the basic application circuit of AD590 for measuring thermodynamic temperature. Because the current flowing through AD590 is proportional to the thermodynamic temperature, when the sum of the resistances of resistor R1 and potentiometer R2 is 1kW, the output voltage VO changes by 1mV/K with temperature. However, due to the gain deviation of AD590 and the error in the resistor, the circuit needs to be adjusted. The adjustment method is as follows: place AD590 in an ice-water mixture and adjust potentiometer R2 to make V_O = 273.2mV. Alternatively, adjust the potentiometer at room temperature (25℃) to make V_O = 273.2 + 25 = 298.2 (mV). However, this adjustment can only guarantee high accuracy near 0℃ or 25℃. 2. The Celsius temperature measurement circuit is shown in Figure 2. Potentiometer R2 is used to adjust the zero point, and R4 is used to adjust the gain of op-amp LF355. The adjustment method is as follows: at 0℃, adjust R2 to make the output V_O = 0, and then at 100℃, adjust R4 to make V_O = 100mV. This adjustment is repeated multiple times until V_O = 0mV at 0℃ and V_O = 100mV at 100℃. Finally, the result is verified at room temperature. For example, if the room temperature is 25℃, then V_O should be 25mV. An ice-water mixture represents a 0℃ environment, while boiling water represents a 100℃ environment. To achieve an output of 200mV/℃ in Figure 2, the feedback resistor (formed by R3 and potentiometer R4 in series) can be increased. Additionally, when measuring Fahrenheit (symbol ℉), since Fahrenheit temperature equals thermodynamic temperature minus 255.4 and then multiplied by 9/5, if an output of 1mV/℉ is required, the feedback resistor should be adjusted to approximately 180kW, resulting in V_O = 17.8mV at 0℃ and V_O = 197.8mV at 100℃. The AD581 is a high-precision integrated voltage regulator with a maximum input voltage of 40V and an output of 10V. 3. Temperature difference measurement circuit and its application (1). Circuit and principle analysis Figure 3 is a circuit that uses two AD590s to measure the temperature difference between two points. With a feedback resistor of 100kW, let the temperatures at AD590s #1 and #2 be t1 (°C) and t2 (°C) respectively, then the output voltage is (t1 - t2)100mV/°C. Potentiometer R2 in the figure is used for zeroing. Potentiometer R4 is used to adjust the gain of the operational amplifier LF355. By Kirchhoff's current law: I + I2 = I1 + I3 + I4 (1) By the characteristics of the operational amplifier, I3 = 0 (2) (3) Adjust the zero-adjustment potentiometer R2 to make I4 = 0 (4) From (1), (2), and (4), we can get: I = I1 - I2 Let R4 = 90kW Then we have: (5) Where (t1 - t2) is the temperature difference in °C. From equation (5), we know that changing the value of (R[sub]3[/sub]+R[sub]4[/sub]) can change the magnitude of V[sub]O[/sub]. (2). Application example Taking a certain energy-saving medicinal material warehouse temperature and humidity control system as an example, if the warehouse temperature is required to be lower than T℃ and the relative humidity is required to be lower than A1B1%RH, then the two control modes adopted are as follows: Control mode one: When the relative humidity inside the warehouse is higher than A1B1%RH and the temperature outside the warehouse is lower than T℃, ventilation is carried out inside and outside the warehouse. This method uses the humidity difference between inside and outside the warehouse to exchange air in order to achieve the dehumidification requirements inside the warehouse. Its advantages are high efficiency, energy saving and cost saving. However, this method is subject to strict control. First, the relative humidity outside the warehouse must be lower than that inside the warehouse, and the difference between them must be greater than A2B2%RH, so as to effectively ensure timely dehumidification inside the warehouse. Secondly, the temperature difference between the inside and outside of the storage room must be less than ΔT℃. This is because if ventilation is carried out when the outside temperature is much higher than the inside temperature, the hot air entering the storage area will encounter cold air, causing condensation on the surfaces of medicines and equipment, thus affecting their quality. Conversely, if ventilation is carried out when the inside temperature is much higher than the outside temperature, the cold air entering the storage room will also cause condensation on the surfaces of medicines and equipment. Additionally, the outside temperature should not approach T℃. This is because if ventilation is carried out when the outside temperature is close to T℃, the temperature inside the sealed storage room may rise, exceeding the upper temperature limit T℃. Control Mode Two: When the temperature is higher than T℃ or the humidity is higher than A1B1%RH but the first condition is not met, the refrigeration air conditioning unit is turned on to cool and dehumidify the storage room. To avoid condensation on the surfaces of medicines and equipment during ventilation due to excessive temperature difference between the inside and outside of the storage room, the accuracy of the system temperature difference value must be strictly controlled. The traditional method of measuring temperature difference involves processing the two temperatures separately (conditioning circuit, A/D, arithmetic processing) and then calculating the difference. This method yields low accuracy in temperature difference measurement. The temperature difference between inside and outside the warehouse can be measured using the circuit shown in Figure 3. By directly comparing the temperature difference value with the set value, high accuracy is ensured, the system software design is simplified, and the system reliability is improved. 4. Measurement of the lowest temperature at N points: Connecting several AD590s at different temperature measurement points in series allows for the measurement of the lowest temperature at all measurement points. This method is applicable to situations requiring the measurement of multiple lowest temperatures. 5. Measurement of the average temperature at N points: Connecting N AD590s in parallel, summing the currents, and averaging the results yields the average temperature. This method is suitable for situations requiring an average temperature at multiple points but not the specific temperature of each point. IV. Conclusion: The specific circuit for measuring thermodynamic temperature, Celsius temperature, two-point temperature difference, multiple lowest temperatures, and multiple average temperatures using the AD590 is widely used in various temperature control applications. Due to its high accuracy, low price, lack of auxiliary power supply, and good linearity, the AD590 is commonly used for temperature measurement and cold junction compensation of thermocouples. References: [1] Wang Furui. Complete Design of Single-Chip Microcomputer Measurement and Control System [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 1998, 282-283. [2] Wang Dahai. 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