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AD590 Based Digital Display Thermometer Design[FAQ]

Author: Mia Date: 15 Dec 2020  375

ad590 circuit

This article will introduce the process of using AD590 to design a high-precision digital thermometer.

 

AD590 is a commonly used T/I converter. It is a current output type two-end temperature sensor made by AD company using the relationship between PN junction forward current and temperature. When the measured temperature is constant, it is equivalent to a constant current source.

AD590 has the characteristics of high measurement accuracy, good linearity and strong interchangeability. When the power supply voltage changes between 5 to 15V, the output current changes less than 1μA. 

Catalog

I. AD590 Introduction

1.1 AD590 Function and Structure

1.2 AD590 Performance Characteristics

1.3 AD590 Working Principle

II. Design of Digital Display Thermometer

2.1 AD590 Internal Circuit

2.2 Design of Temperature Measurement Circuit

2.3 A/D Conversion and Display Circuit Design

III. Conclusion

FAQ

Ordering & Quantity

 

I. AD590 Introduction

 

1.1 AD590 Function and Structure

 

AD590 is a current-type temperature sensor that uses current as an output to indicate temperature. According to the different characteristics, AD590 uses the suffix I, J, K, L, M to divide gears. AD590L and AD590M are generally used in precision temperature measurement circuits. The appearance structure is shown in Figure 1 below. It is packaged in a metal round shell with 3 pins, where pin 1 is the positive terminal of the power supply "+"; pin 2 is the current output terminal "-"; 3 The pin is the ground terminal of the tube shell and is generally not used. The circuit symbol is shown in Figure 2 below.

Figure 1 The metal round shell package structure of AD590

Figure 1 The metal round shell package structure of AD590

Figure 2 The circuit symbol of AD590

Figure 2 The circuit symbol of AD590

 

1.2 AD590 Performance Characteristics

(1) Wide working voltage range: 4~30V;

(2) Wide temperature measurement range: -55~+150℃;

3) Linear current output: 1μA/K,

(4) Excellent linearity: the non-linear error in the temperature measurement range is less than ±0.3℃ (AD590M);

(5) Laser fine-tuning makes the calibration temperature reach: ±0.5℃ (AD590M);

(6) Maximum forward voltage: +44V;

(7) Maximum reverse voltage: -20V;

(8) Storage temperature: -65~+175℃.

 

1.3 AD590 Working Principle

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-value resistor in series at the output end. Then, the current flowing through this resistor will be The measured temperature is proportional, and there will be a voltage signal of 1mV/K across the resistor. Its basic circuit is shown as in Fig. 3.

Figure 3 The core circuit of the temperature sensing part

Figure 3 The core circuit of the temperature sensing part

Figure 3 is the core circuit of the temperature sensing part of the integrated PN junction sensor using the ΔUBE 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 transistors for temperature sensing. The materials and processes of the two tubes are exactly the same, but T3 is essentially It is made up of n transistors in parallel, so its junction area is n times that of T4. The emission junction voltages UBE3 and UBE4 of T3 and T4 are connected in series with the reverse polarity to the resistor R, so the upper end voltage of R is ΔUBE. Therefore, the current I1 is:

 

  I1=ΔUBE/R=(KT/q)(lnn)/R

 

For AD590, n=8, in this way, the total current of the circuit will be proportional to the thermodynamic temperature T, and lead this current to the load resistance RL to get an output voltage proportional to T. Due to the use of constant current characteristics, the output signal is not affected by the power supply voltage and wire resistance. The resistor R in Figure 3 is a thin-film resistor formed on a silicon board. The resistor has been laser-corrected for its resistance value, so an I value of 1μA/K can be obtained at the reference temperature.

 

II. Design of Digital Display Thermometer

 

2.1 AD590 Internal Circuit

 

Figure 4 shows the internal circuit of AD590. VT1 to VT4 in the figure are equivalent to VT1 and VT2 in Figure 3, and VT9 and VT11 are equivalent to VT3 and VT4 in Figure 3. R5 and R6 are low temperature coefficient resistors made by thin film technology. The function of VT6 is to balance the collector voltage of VT7 and VT8.

Figure 4 The internal circuit of AD590

Figure 4 The internal circuit of AD590

 

VT5, VT12 and VT10 are start-up circuits, and VT5 is a constant bias diode. VT10 is not connected to the substrate, but is connected to R3 to isolate the substrate capacitance and prevent the substrate capacitance from affecting the frequency stability.

 

VT6 can also be used to prevent damage to the circuit when the power supply is reversed. R1 and R2 are emitter feedback resistors, which can be used to increase impedance. VT1~VT4 are connection methods designed for thermal effects. C1 and R4 are used to prevent parasitic oscillation. The design of the circuit makes the emitter currents of VT9, VTl0, and VT11 equal, and they are also 1/3 of the total current I of the entire circuit.

The launch junction area ratio of VT9 and VT11 is 8:1, and the launch junction area of VT10 and VT11 is equal.

 

The emitter junction voltages of VT9 and VT11 are connected in series with opposite polarity to the resistors R5 and R6, so: △UBE=(R6-2R5)I/3, from the above formula, we can see that increasing R5 and decreasing R6 will make △ UBE decreases, but the effect of changing R5 on △UBE is more significant because the coefficient in front of it is larger. In the production process, laser correction R5 is used for coarse adjustment, and R6 is corrected for fine adjustment, and finally its output current l is 1μA/K.

2.2 Design of Temperature Measurement Circuit

 

Because AD590 is a current output device, when designing a temperature measurement circuit, first convert current into voltage. When the temperature increases by 1K, the current of the AD590 increases by 1μA. When the output current of AD590 passes through a 10kΩ resistor, the voltage drop on this resistor is 10mV, which is converted into 10mV/K. In order to make this resistance accurate to 0.1%, a 9.6kΩ resistor and a 1kΩ precision potential can be used,.

 

The device is connected in series, and a precise 10kΩ resistance is obtained by adjusting the precision potentiometer. Figure 5 shows a current→voltage and absolute→Celsius temperature scale conversion circuit, in which the operational amplifier A1 is connected to the form of a voltage follower to increase the input impedance of the signal. The function of the operational amplifier A2 is to convert the absolute temperature scale into a Celsius temperature scale, input a constant voltage (such as 1.365V) to the non-inverting input terminal of A2, and then adjust RP2 to amplify this voltage to 2.730V. In this way, the voltage between the output terminals of A1 and A2 is the voltage corresponding to the converted Celsius temperature scale.

Figure 5 Current→Voltage and Absolute→Celsius Temperature Conversion Circuit.

Figure 5 Current→Voltage and Absolute→Celsius Temperature Conversion Circuit

 

Put AD590 into the ice-water mixed solution at 0℃, adjust RP1 so that the output voltage of A1 is 2.730V, adjust RP2 so that the output voltage of A2 is also 2.730V, so the voltage between the two output terminals of A1 and A2: 2.730-2.730 =0V, which corresponds to 0°C.

2.3 A/D Conversion and Display Circuit Design

 

There are two schemes for designing A/D conversion and display circuits:

 

(1) Realize with A/D converter MC14433

 

First, the output current of AD590 is converted 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 analog signals into digital signals through the A/D converter MC14433 to control the display circuit. Among them, MC14511 is a decoding/latch/drive circuit, its input is BCD code, and its output is a seven-segment decoding. The LED digital display is driven by the MC14433 bit selection signals DS1~DS4 through the Darlington array MC1413, and the DS1 and Q2 terminals of the MC14433 control the display of "+" and "-" temperature. When DS1=1 and Q2=1, the display is positive; when Q2=0, the display is negative.

Figure 6 Block diagram of A/D conversion and digital display circuit

Figure 6 Block diagram of A/D conversion and digital display circuit

 

(2) Realize with ICL7106

 

A/D conversion and LCD display circuit block diagram using ICL7106 is shown in Figure 7. Among them, ICL7106 is a 3 and a half display A/D conversion circuit, which contains a liquid crystal display drive circuit, which can be used for A/D conversion and LCD display drive.

Figure 7 A/D conversion and LCD display circuit block diagram

Figure 7 A/D conversion and LCD display circuit block diagram

III. Conclusion

AD590 has the advantages of excellent linearity, stable performance, high sensitivity, no compensation, small heat capacity, strong anti-interference ability, remote temperature measurement and convenient use. It can be widely used in various temperature measurement and control fields such as refrigerators, air conditioners, granaries, ice storages, and industrial equipment. The digital display thermometer designed in this article based on AD590 has been applied in many 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.

  1. 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°.
  2. 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.
  3. 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.

Ordering & Quality

Photo Mfr. Part # Company Description Package PDF Qty Pricing
(USD)
AD590JH AD590JH Company:Analog Devices Inc. Remark:SENSOR ANALOG -55C-150C TO52-3 Package:TO-206AC, TO-52-3 Metal Can
DataSheet
In Stock:2463
Inquiry
Price:
1+: $19.76000
10+: $18.22700
25+: $17.40800
100+: $15.56480
Inquiry
AD590JR AD590JR Company:Analog Devices Inc. Remark:Temperature Sensor Analog, Local -55°C ~ 150°C 8-SOIC Package:8-SOIC (0.154"", 3.90mm Width)
DataSheet
In Stock:On Order
Inquiry
Price:
Call
Inquiry
AD590KH AD590KH Company:Analog Devices Inc. Remark:SENSOR ANALOG -55C-150C TO52-3 Package:TO-52
DataSheet
In Stock:2189
Inquiry
Price:
1+: $21.31000
10+: $19.65100
25+: $18.76800
100+: $16.78080
Inquiry
AD590LF AD590LF Company:Analog Devices Inc. Remark:SENSOR ANALOG -55C-150C 2FLATPK Package:2-CFlatpack
DataSheet
In Stock:833
Inquiry
Price:
1+: $69.25000
10+: $65.78600
25+: $64.05440
Inquiry
AD590MF AD590MF Company:Analog Devices Inc. Remark:SENSOR ANALOG -55C-150C 2FLATPK Package:2-CFlatpack
DataSheet
In Stock:41
Inquiry
Price:
1+: $117.72000
10+: $115.33300
Inquiry

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