In order to facilitate the rapid selection of the majority of research and development engineers, accelerate the progress of R & D, this paper provides a circuit for the signal conditioning section for reference. The hand-held infrared thermometer typically uses an infrared thermal stain sensor (INFRARED thermostile sensor), which is often used in non-touchless infrared temperature measurements because any objects above absolute zero can emit infrared rays, and the energy radiated from the object As the surface temperature of the object is increased, the thermopile sensor can measure the energy radiated from the object and accurately measure the temperature of the surface of the object.
Because each change of the measured object, the thermoelectric stack sensor output only increases 50UV ~ 100UV, so that the output voltage of the 0.1 ° C resolution is only 5UV ~ 10UV, so the thermoelectric stack probe is selected preferably with high sensitivity. In addition, according to different temperature measurement distances, the probes of different viewing angles are selected because the voltage value of the actual output of the thermoelectric stack probe is the average temperature value of the object in the entire field of view. In addition, in order to improve the signal-to-noise ratio of the thermoelectric stack probe, the resistance value of the thermoelectric stack is also one of the key factors. The larger the resistance of the resistance, the output thermal noise is more larger.
It is because the signal output from the thermoelectric stack sensor is so weak, so that the amplified voltage error needs to be small, only zero drift self-steady type of op amp can meet the zoom conditioning of the signal. There is a master amplifier inside this type of op amp, and an auxiliary amplifier automatically calibrates the offset voltage at a time interval at the US level to ensure that the offset voltage is minimum, especially for the amplification conditioning of low frequency signals.
It should be noted that the larger the offset voltage, the greater the incoming error, and if you want to calibrate the influence of the offset voltage, it is undoubtedly to increase the design complexity of hardware, and will increase the process and complexity of the production line processing, and the cost is also Increase and reduce productivity. It is important to note that if the accuracy of the average gun is ± 0.3 ° C, the voltage generated by the thermal stack at 0.3 ° C is only 15UV ~ 30UV, meaning that the maximum offset voltage value of the op amp cannot be greater than 15UV ~ 30UV . Since the thermoelectric stack sensor is about 100 kΩ, it belongs to a relatively large resistance, which requires an input bias current that the op amp must be a PA level to reduce the error, so the op amp can only be a CMOS architecture.
The following figure is a typical application circuit of zero-tempered DS8551 in hand-held temperature gun, and R & D engineers can do it as a design reference. The DS8551 input offset voltage is less than 3UV, DC gain is up to 140 dB, which is ideal for such applications.
Application points:
1. R1, R2 constitutes a feedback circuit, its magnification is BETA = R2 / R1 = 510. The feedback resistor R2 needs to be connected in parallel, limiting the bandwidth of the entire circuit within 10 Hz, sufficiently reduces the output of the amplifying circuit. noise. The most important thing is 1 / F noise of op amp; reducing C3 can increase the corresponding time of op amp, but will introduce more noise, need to apply the engineer to pay attention. R4 takes R4 = R1.
2. Output VOUT after the RC filtering of the output ends R3 and C5. Because the DS8551 op amp is compensated, R3 cannot be too small, otherwise C5 = 100nf will introduce a low frequency pole, it is possible to cause the output of the output.
3. The DS8551 can power the power supply terminal + VS by R6 and C1, R6 is not too large, otherwise it affects the power supply capacity.
4. R5 Value is taken according to the specific parameters of the NTC in the thermoelectric stack, and the thermal stack PIN2 outputs temperature compensation to the MCU ADC sample.
5. Considering the maximum range of op amp, VBIAS recommends VCC of 1/2, can be implemented by resistance, or can be connected to a specific reference VREF, VREF recommended range 0.3VCC ~ 0.7VCC
6. VCC scope suggestion is 3V ~ 5V
7. Vout gives the MCU's ADC sample, formula is VDIFF = (VTP +) - (VOTA + VBIAS) / (Beta + 1), according to the VDIFF value and NTC temperature, the table gets the temperature detected by the object . Since the op amp has enlarged small signals, it reduces the accuracy requirements of the ADC of the MCU.
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Reprinted from -Wiku Electronic Market Network
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