AD590 temperature measurement circuit

AD590 temperature measurement circuit, AD590 is a monolithic integrated two-terminal temperature-sensing current source produced by Analog Devices, Inc. of the United States. 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, that is:
=1
T
IT mA/K
Where: TI — current flowing through the device (AD590), unit is mA;
T — thermodynamic temperature, unit is K.
2. The temperature measurement range of AD590 is -55℃～+150℃.
3. The power supply voltage range of AD590 is 4V～30V. The power supply voltage can
vary in the range of 4V~6V, and the current TI changes by 1mA, which is equivalent to a temperature change of 1K. AD590 can
withstand 44V forward voltage and 20V reverse voltage, so the device will not be damaged if it is reversely connected.
4. The output resistance is 710MW. 1 Functions and characteristics of AD590
　　AD590 is a current-type temperature sensor. The required temperature value can be obtained by measuring the current. According to the characteristic classification, the suffix of AD590 is represented by I, J, K, L, and M. AD590L and AD590M are generally used in precision temperature measurement circuits. The circuit appearance is shown in Figure 1. It uses a metal shell 3-pin package, where pin 1 is the positive power supply terminal V+; pin 2 is the current output terminal I ₀ ; and pin 3 is the tube shell, which is generally not used. The circuit symbol of the integrated temperature sensor is shown in Figure 2.

    The main characteristic parameters of AD590 are as follows: working voltage: 4~30V;     working temperature: -55~+150℃; storage temperature: -65~+175℃; forward voltage: +44V; reverse voltage: -20V;     welding temperature (10 seconds): 300℃; sensitivity: 1μA/K.  
    

    
    
    

    
2 Working principle of AD590
　　When the measured temperature is constant, AD590 is equivalent to a constant current source. Connect it to a 5~30V DC power supply and connect a 1kΩ constant resistor in series at the output end. Then, the current flowing through this resistor will be proportional to the measured temperature. At this time, there will be a 1mV/K voltage signal at both ends of the resistor. Its basic circuit is shown in Figure 3.

　　Figure 3 is the core circuit of the temperature sensing part of the integrated PN junction sensor using the ΔU _BE characteristic. Among them, T1 and T2 play a constant current role, which can be used to make the collector currents I1 and I2 of the left and right branches equal; T3 and T4 are temperature sensing transistors. The materials and processes of the two tubes are exactly the same, but T3 is actually made up of n transistors in parallel, so its junction area is n times that of T4. The emitter junction voltages U _BE3 and U _BE4 of T3 and T4 are added to the resistor R after being connected in series with reverse polarity, so the voltage on the upper end of R is ΔU _BE . Therefore, the current I1 is: I ₁ = ΔU _BE / R = (KT / q) (lnn) / R 　　For AD590, n = 8, so the total current of the circuit will be proportional to the thermodynamic temperature T. This current can be directed to the load resistor RL _to obtain an output voltage proportional to T. Since the constant current characteristic is used, the output signal is not affected by the power supply voltage and wire resistance. The resistor R in FIG3 is a thin film resistor formed on a silicon plate, and its resistance value has been corrected by laser, so that an I value of 1 μA/K can be obtained at the reference temperature.
    

AD590 internal circuit

　　Figure 4 shows the internal circuit of AD590. T1~T4 in the figure are equivalent to T1 and T2 in Figure 3, while T9 and T11 are equivalent to T3 and T4 in Figure 3. R5 and R6 are low temperature coefficient resistors made by thin film technology, which are used for factory adjustment. T7, T8, and T10 are symmetrical Wilson circuits used to increase impedance. T5, T12, and T10 are startup circuits, of which T5 is a constant bias diode.
　　T6 can be used to prevent damage to the circuit when the power supply is reversed, and it can also make the left and right branches symmetrical. R1 and R2 are emitter feedback resistors, which can be used to further increase impedance. T1~T4 is a connection method designed for thermal effects. C1 and R4 can be used to prevent parasitic oscillation. The design of this circuit makes the emitter currents of T9, T10, and T11 equal and 1/3 of the total current I of the entire circuit. The emitter junction area ratio of T9 and T11 is 8:1, and the emitter junction areas of T10 and T11 are equal.
　　The emitter junction voltages of T9 and T11 are connected in series with opposite polarities and then added to resistors R5 and R6, so we can write: ΔU _BE = (R ₆ -2 R ₅ ) I/3. 　　R6 only has the emitter current of T9, while R5 has the emitter current from T11 in addition to the emitter current from T10, so the voltage drop on R5 is 2/3 of R5. 　　It is not difficult to see from the above formula that if you want to change ΔU _BE , you can adjust R5 and then R6, and the effect of increasing R5 is the same as reducing R6, and the result will reduce ΔU _BE . However, the effect of changing R5 on ΔU _BE is more significant because its coefficient is larger. In fact, laser is used to correct R5 for coarse adjustment and R6 for fine adjustment, so that the total current I reaches 1μA/K under 250℃.
    


3 Design of digital display thermometer
AD590 has the advantages of excellent linearity, stable performance, high sensitivity, no need for compensation, small heat capacity, strong anti-interference ability, long-distance temperature measurement and easy use. It can be widely used in various refrigerators, air conditioners, granaries, ice storage, industrial instrument matching and various temperature measurement and control fields. The 　　following is the design process of using AD590 to form a digital display thermometer. 3.1 Design of temperature measurement circuit 　　When designing a temperature measurement circuit, the current should be converted into voltage first. Since AD590 is a current output element, its current increases by 1μA for every 1K increase in temperature. When the current of AD590 passes through a 10kΩ resistor, the voltage drop on this resistor is 10mV, which is converted to 10mV/K. In order to make this resistor accurate (0.1%), a 9.6kΩ resistor can be connected in series with a 1kΩ potentiometer, and then the potentiometer can be adjusted to obtain an accurate 10kΩ. Figure 5 shows a current/voltage and absolute/Celsius temperature conversion circuit, in which operational amplifier A1 is connected in the form of a voltage follower to increase the input impedance of the signal. The function of operational amplifier A2 is to convert the absolute temperature scale into the Celsius temperature scale, input a constant voltage (such as 1.235V) to the in-phase input terminal of A2, and then amplify this voltage to 2.73V. In this way, the voltage between the output terminals of A1 and A2 is the converted Celsius temperature scale.　　

　　When AD590 is placed in a 0℃ ice-water mixture, the voltage at the in-phase input of A1 should be 2.73V, and the output voltage of A2 should also be 2.73V. Therefore, the voltage between the output terminals of A1 and A2 is:
　　2.73-2.73=0V, which corresponds to 0℃.
3.2 Design of A/D conversion and display circuit
　　There are two schemes for designing A/D conversion and display circuit. They are described as follows:
    (1) Implementation with A/D converter MC14433
　　First, convert the output current of AD590 into voltage. Since this signal is an analog signal, it is necessary to convert this signal into a digital signal for digital display. The conversion circuit using MC14433 is shown in Figure 6. The function of this circuit is to convert the analog signal into a digital signal through the A/D converter MC14433 to control the display circuit. Among them, MC14511 is a decoding/latch/driving circuit, whose input is BCD code and output is seven-segment decoding. The LED digital display is driven by the bit selection signals DS1~DS4 of MC14433 through the Darlington array MC1413, and the DS1 and Q2 terminals of MC14433 control the display of "+" and "-" temperature. When DS1=1, Q2=1, the display is positive; when Q2=0, the display is negative.

Figure 6 A/D conversion and digital display circuit block diagram

    (2) Using ICL7106 to implement
　　The block diagram of the A/D conversion and LCD display circuit using ICL7106 is shown in Figure 7. Among them, ICL7106 is a 3.5-bit display A/D conversion circuit, which contains a liquid crystal display drive circuit and can be used for A/D conversion and LCD display drive.

4 Conclusion
　　Temperature sensors have a wide range of applications. They are not only widely used in daily life, but also widely used in automation and process detection control systems. There are
　　many types of temperature sensors. According to the on-site conditions, choosing the appropriate sensor type can ensure accurate and reliable measurement, and at the same time achieve the effect of increasing service life and reducing costs.