Oct 16 2020

# AD590 Based Temperature Measurement Circuit

## I. Introduction

AD590 is a current output type two-end temperature sensor made by AD company using the relationship between PN junction forward current and temperature. Because the device has good linear characteristics and interchangeability, it has high measurement accuracy and has the characteristics of eliminating power fluctuations. This text analyzes the advantages of AD590, and uses the energy-saving temperature and humidity control system as an example to introduce the application of using AD590 to measure two-point temperature difference circuit.

## Catalog

 I. Introduction II. AD590 Advantages and Features III. Celsius Temperature Measurement Circuit IV. Temperature Difference Measurement Circuit and Its Application4.1 Circuit and Principle Analysis4.2 Application Examples4.3. Measurement of the Lowest Temperature Value at Point N4.4. Measurement of Average Temperature at Point N V. Conclusion FAQ

AD590 type current output integrated temperature sensor, the domestic similar product model is SG590. In practice, the corresponding temperature value can be obtained by measuring the current. The suffix of AD590 is represented by I, j, K, L, M, which essentially refers to different characteristics and different measurement temperature ranges. Its advantages and characteristics are as follows:

(1) Linear output current: 1 µA/K

(2) Wide temperature range: -55°C to +150°C

(3) Ceramic sensor package compatible with probe

(4) Two-terminal device: voltage input/current output

(6) Excellent linearity: full scale range ±0.3°C (AD590M)

(7) Wide power supply voltage range: 4 V to 30 V

(8) The sensor is insulated from the housing

(9) Low cost

## III. Celsius Temperature Measurement Circuit

AD590 is a current output integrated temperature sensor. When designing a temperature measurement circuit, the current must be converted into a voltage. For every 1K increase in temperature, the current increases by l µA. The design of the Celsius temperature measurement circuit must complete two tasks: one is to convert the current output by the AD590 into a voltage signal, that is, the current is converted into a voltage circuit. The second is to convert the thermodynamic temperature into Celsius, that is, the absolute temperature is converted to Celsius. The working principle of the Celsius temperature measurement circuit is shown in Figure 1.

According to the characteristics of AD590, for every lK thermodynamic temperature increased, the current increases by luA, when the load resistance is 10KΩ, the voltage drop on this resistance is 10mV.

Among them, AD590, potentiometers RPl and R1, and operational amplifier A1 form a current-to-voltage conversion circuit. A1 is connected in the form of a voltage follower, mainly to increase the input resistance of the signal.

The operational amplifier A2 is the core device that converts absolute temperature to Celsius. Its conversion principle is that zero Celsius corresponds to 273K thermodynamics. Therefore, the reference voltage must be set to convert thermodynamics to Celsius humidity. The value is 2.73V corresponding to zero Celsius. The realization method is to input a constant voltage to the end of the same name of A2. The constant voltage is provided by the current limiting resistor R2 and the Zener tube. The constant voltage selection Zener tube model is CW385 with a value of 1.235V. A2 amplifies this voltage to 2.73 v, RP2 is to adjust the gain of A2 operational amplifier.

Through the conversion circuit, the voltage at the output terminals of A1 and A2 is the voltage value proportional to the temperature in degrees Celsius, that is, the voltage value corresponding to 100mV per degree Celsius.

Special note: When debugging, put the integrated temperature sensor AD590 in the zero-degree ice water solution, first adjust the RPl potentiometer to make the A1 operational amplifier output 2.73V, and then adjust the RP2 potentiometer to make the A2 operational amplifier output 2.73V, Therefore, the output voltage of the temperature measurement circuit is 0V at zero degrees Celsius. The changing law is that every degree Celsius corresponds to an output voltage of 10mV.

Figure 1. Celsius temperature measurement circuit

## IV. Temperature Difference Measurement Circuit and Its Application

### 4.1 Circuit and Principle Analysis

Figure 2 is a circuit that uses two AD590s to measure the temperature difference between two points. In the case of feedback resistance of 100kW, set the temperature at 1# and 2# AD590 as t1(℃) and t2(℃) respectively, then the output voltage is (t1-t2)100mV/℃. The potentiometer R2 in the picture is used for zero adjustment. Potentiometer R4 is used to adjust the gain of the op amp LF355.

Figure 2. Circuit for measuring temperature difference between two points

From Kirchhoff’s current law: I+I2=I1+I3+I4 (1)

Known from the characteristics of the operational amplifier: I3=0 (2)

(3)

adjust zero potentiometer R2 so that: I4=0 (4)

From (1), (2), (4), we can get: I=I1-I2

Setup: R4=90kW

then: Vo = I(R3+R4) = (I1-I2) (R3+R4) =(t1-t2)100mV/℃ (5)

Among them, (t1-t2) is the temperature difference, the unit is °C.

Knowing from formula (5), changing the value of (R3+R4) can change the size of VO.

### 4.2 Application Examples

Take 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℃, the relative humidity is lower than A1B1%RH. The two control modes adopted are as follows:

Control mode 1: When the relative humidity in the warehouse is higher than A1B1%RH and the temperature outside the warehouse is lower than T℃, ventilation inside and outside the warehouse is performed. This method uses the difference in humidity inside and outside the warehouse to exchange air to meet the requirements of dehumidification in the warehouse. Its advantages are high efficiency, energy saving, and money saving. 、

However, this method is strictly controlled. First of all, the relative humidity outside the warehouse should be lower than that in 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 warehouse should be less than △T℃. 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 entering the warehouse area will encounter cold air, which will cause condensation on the surface of the medicines and equipment, then affects the their quality.

Conversely, if ventilation is carried out when the temperature inside the warehouse is much higher than the temperature outside the warehouse, cold air will also condense on the surface of the medicine equipment after entering the warehouse.

In addition, the outside temperature cannot be close to T°C. This is because if ventilation is performed when the temperature outside the storage is close to T°C, the temperature of the closed storage is likely to rise, thereby exceeding the upper temperature limit T°C.

Control mode 2: When the temperature is higher than T℃ or the humidity is higher than A1B1%RH, but the first condition is not met, the refrigerating and air-conditioning unit is turned on to cool and dehumidify in the warehouse.

In order to avoid the phenomenon of condensation on the surface of medicines and equipment due to the excessive temperature difference between the inside and outside of the warehouse, the accuracy of the system temperature difference must be strictly controlled.

The traditional method of measuring the temperature difference is to process the temperature of the two points separately (conditioning circuit, A/D, arithmetic processing) and then find the difference. This method has low accuracy of the temperature difference. The temperature difference measurement inside and outside the library can use the circuit shown in Figure 2, using the temperature difference value to directly compare with the set value, which can ensure higher accuracy, simplify the software design of the system, and improve the reliability of the system.

### 4.3. Measurement of the Lowest Temperature Value at Point N

Connect several AD590s at different temperature measuring points in series, and the lowest temperature value at all measuring points can be measured. This method can be applied to the occasion of measuring the lowest temperature at multiple points.

### 4.4. Measurement of Average Temperature at Point N

Connect N AD590s in parallel, and then average the current after summing, then the average temperature can be obtained. This method is suitable for the occasions where the average temperature of multiple points is required but the specific temperature of each point is not required.

## V. Conclusion

AD590 integrated temperature sensor is widely used. It is mainly used in engineering to measure thermodynamic temperature, Celsius temperature, temperature difference between two points, minimum temperature at multiple points, average temperature at multiple points, etc. Therefore, it is not only widely used in daily life, but also widely used in industrial automation control systems and automatic detection process control systems. In addition, due to its high accuracy, low price, no auxiliary power supply, and good linearity, AD590 is often used in temperature measurement and temperature detection and control fields.

## FAQ

 What is AD590? AD590 is a temperature sensor, the current output sensitivity is 1μA/℃, the standard output value is 298.2μA at 25℃, and the working voltage range is 4~30V. What are the characteristics of AD590 temperature sensor? Single function (only temperature measurement), small temperature measurement error, low price, fast response speed, long transmission distance, small size, micro power consumption, etc. It is suitable for remote temperature measurement and temperature control without non-linear calibration. The peripheral circuit is simple. How to detect the quality of AD590? AD590 has a current of 273 mA at 0°. Because 2113 is a Wen sensitive resistor 5261, it means that it is greatly affected by the surrounding temperature 4102. It is very difficult to measure without relying on 1653 other tools. Give you some suggestions. When the ambient temperature rises by one degree, the current of AD590 increases by 1uA. What you have to do is to work with AD590 simultaneously with the help of a high-precision temperature test instrument. After AD590 series 10K resistance, measure its voltage, that is to say, it should be 2.73V at 0°, and 2.98V at room temperature 25°. For higher accuracy, it is recommended that you use the electronic building block software Ardunio for measurement, and put the corresponding data into MATLAB for linear regression. The better the linearity, the more stable the measurement. AD590 is not a high-precision temperature testing device. If high-precision testing is required, other components are recommended. What is the difference between AD590 and PT100? AD590 is a current-type temperature sensor. It converts temperature changes into current conversion. The simplest processing is to pass a resistor (10K) after the output to convert the current into a voltage, and then through the detection voltage, the current at this time can be deduced. Use the relationship between current and temperature in the sensor data to calculate the current temperature. PT100 is a resistance type temperature sensor, which converts temperature changes into resistance changes. The simplest process is to place Pt100 in a bridge, use the voltage difference at the midpoint of the bridge arm, and use a differential amplifier circuit (instrument amplifier circuit) Amplify the voltage, use the amplifier gain and bridge structure data, and use the detected voltage to inversely calculate the current resistance value, and use the relationship between resistance and temperature in the PT100 data sheet to calculate the current temperature. Is AD590 a thermocouple or a thermal resistance? It is neither a thermocouple nor a thermal resistance. The main principle is to detect the temperature according to the temperature change, the output current change, and the current size.