The AD590 temperature sensor is a monolithic integrated two-terminal temperature sensing current source from the American ANALOG DEVICES company. Its output current is proportional to the absolute temperature. The device operates as a high-impedance, constant-current regulator with a regulation factor of 1 µA/K over a supply voltage range of 4 V to 30 V. The on-chip thin film resistors are laser trimmed and used to calibrate the device so that it outputs 298.2 µA at 298.2K (25°C).

　　The AD590 temperature sensor is suitable for any temperature sensing application below 150°C where traditional electrical temperature sensors are currently used. The low cost of a single-chip integrated circuit and the lack of support circuitry make it an attractive alternative for many temperature measurement applications. When applying the AD590, there is no need for linearization circuits, precision voltage amplifiers, resistance measurement circuits and cold junction compensation. In addition to temperature measurement, it can also be used for temperature compensation or correction of discrete devices, bias proportional to absolute temperature, flow rate measurement, liquid level detection, wind speed determination, etc. The AD590 temperature sensor is available in bare die form and is suitable for hybrid circuits and fast temperature measurements in protected environments.

　　The AD590 temperature sensor is particularly suitable for remote sensing applications. It provides a high impedance current output and is insensitive to voltage drops on long lines. Any well-insulated twisted pair will work, up to several hundred feet away from the receiving circuit. This output characteristic also facilitates multiplexing of the AD590 temperature sensor: the output current can be switched through a CMOS multiplexer, or the supply voltage can be switched through a logic gate output.

AD590 temperature sensor AD590HJ temperature measuring element

　　AD590 temperature sensor application

　　AD590 temperature sensor is a specific circuit for measuring thermodynamic temperature, Celsius temperature, temperature difference between two points, minimum temperature at multiple points, and average temperature at multiple points. It is widely used in different temperature control occasions. Because the AD590 temperature sensor has high accuracy, low price, no auxiliary power supply, and good linearity, it is often used for temperature measurement and cold-junction compensation of thermocouples. Application example: Take an energy-saving medicinal material warehouse temperature and humidity control system as an example. If the warehouse temperature is required to be lower than T°C, the relative humidity is lower than A1B1%RH. The two control modes adopted are as follows:

　　AD590 temperature sensor control mode 1: When the relative humidity inside the library is higher than A1B1%RH and the temperature outside the library is lower than T℃, ventilation inside and outside the library is performed. This method uses the humidity difference between the inside and outside of the warehouse to exchange air to meet the dehumidification requirements in the warehouse. Its advantages are high efficiency, energy saving, and money saving. But this approach is strictly controlled. First of all, the relative humidity outside the warehouse should be lower than that inside the warehouse, and the difference between them should 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 warehouse should be less than △T°C. This is because if ventilation is performed when the temperature outside the warehouse is much higher than the temperature inside the warehouse, the hot air will enter the warehouse area and encounter cold air, which will cause damage to medicines and equipment. The phenomenon of surface condensation affects the quality of medicines and equipment. On the contrary, if ventilation is performed when the temperature inside the warehouse is much higher than the temperature outside the warehouse, condensation will form on the surface of the pharmaceutical equipment after the cold air enters the warehouse. In addition, the temperature outside the library cannot be close to T℃. This is because if ventilation is performed when the outside temperature is close to T°C, it is likely to increase the temperature of the closed warehouse and exceed the upper temperature limit T°C.

　　AD590 temperature sensor control mode 2: When the temperature is higher than T℃ or the humidity is higher than A1B1%RH but does not meet the first condition, the refrigeration air conditioning unit is turned on to cool down and dehumidify the warehouse. In order to avoid condensation on the surface of medicines and equipment during ventilation due to excessive temperature differences between inside and outside the warehouse, the accuracy of the system temperature difference must be strictly controlled. The traditional temperature difference measurement method is to process the two point temperatures separately (conditioning circuit, A/D, arithmetic processing) and then find the difference. The accuracy of the temperature difference obtained by this method is low. The temperature difference measurement inside and outside the warehouse can use the circuit shown in Figure 3. The temperature difference value is directly compared with the set value, which can not only ensure higher accuracy, but also simplify the system software design and improve the reliability of the system.