Thermocouple signal acquisition circuit analysis and optimization method1

In the field of iron and steel metallurgy, temperature is a very important parameter, thermocouple with high accuracy, fast response time, wide temperature range, the advantages of low cost and long service life, and be the first choice of the iron and steel metallurgy field temperature detecting element. Thermocouple temperature measurement accuracy is of vital significance for industrial production process control, the influence factors in many ways the thermocouple temperature measurement precision, besides the influence of thermoelectric material, installation position is right, whether reasonable installation method, thermocouple compensation wire material, such as side also include secondary collection element ( Such as thermocouple secondary instrument, PLC thermocouple module, etc. ) The factors, such as thermocouple signal acquisition circuit, signal nonlinear processing technology and appropriate cold junction compensation method, etc. This article emphatically from the above three factors analysis of thermocouple signal acquisition accuracy, and reasonable optimization method is put forward.

1, the basic working principle of thermocouple

the two different material a conductor or semiconductor ( A and B) Welded together to form a closed loop, the closed loop is called thermoelectric loop. When the two junction at different temperature T1 and T0, loop will generate electromotive force, this kind of phenomenon called the thermoelectric effect, and the electromotive force called thermoelectric potential. Thermocouple is to use the principle of temperature measurement, which directly used for measuring the medium temperature is called at the end of the working side ( Also known as measuring side) , on the other side called cold end ( Also known as compensation) And the cold end connected to the measuring instrument. When the hot end and cold end temperature, measuring instrument can measure the temperature of the measured medium. The thermocouple thermoelectric potential increases along with the rise of temperature, the size of the thermoelectric potential associated with the material, at the ends of the thermocouple temperature of thermocouple, and has nothing to do with the hot electrode length and diameter. Working principle of the thermocouple is shown in figure 1.

2 thermocouple signal acquisition precision analysis

2. 1 thermocouple signal collection circuit analysis

thermocouple signals is weak microvolt level of slow change, the signal in the process of transmission, susceptible to electromagnetic interference of the outside world, so the thermocouple thermometer table to collect the thermocouple signal, the signal front need to increase the signal conditioning circuit, improve the electromagnetic anti-interference ability, and signal collection to choose high precision, high reliability of the AD converter, AD converter reference voltage source at the same time with high accuracy, low drift reference voltage source, and more need to adopt differential thermocouple signal input to eliminate most of the common mode noise on thermocouple wire, through the above appropriate circuit optimization design can ensure the accuracy requirement of the thermocouple signal acquisition.

2。 2 thermocouple signal acquisition circuit optimization method

thermocouple signal collection optimization solution from the following three aspects:

the front-end design difference signal conditioning circuit signal to a low-pass filter and common mode filter, the filter can be used in a passive components such as resistors and capacitors, the cut-off frequency of filter determines the resistance capacity value, also can consider to use active filter, low imbalance, low temperature drift of the operational amplifier is adopted to form active filter, and operational amplifier high input impedance, output impedance is low, can provide good isolation performance, and can provide the required gain.

AD converter selection AD converters, appropriate chooses high resolution, low noise, high common mode rejection ratio, with differential input channels, built-in programmable gain AD converter. The higher the resolution, the higher the acquisition accuracy, but the device cost will increase, acquisition system should choose appropriate according to the signal fluctuation of the minimum resolution. The thermocouple S index, for example, the change 1 ℃ the minimum is 5 & mu; V, the least significant bit LSB must be less than 5 & mu; V。

AD converter reference voltage source AD converter of the benchmark voltage source can choose internal or external alone pick up a reference voltage source, voltage reference source for low temperature drift, high precision, low noise. Built-in precision reference voltage source, to higher precision reference voltage source, is asked to select the external voltage source.

in AD7793 precise measuring thermocouple signals to illustrate. AD7793 application is suitable for high accuracy measurement of low power consumption, low noise, complete analog front end, a built-in low noise, 16 with three differential analog input & Sigma; - Δ ADC, it integrates on chip low noise instrument amplifier, and gain can be set up.

AD7793 measuring thermocouple signal circuit is shown in figure 2. AD7793 the differential input is used to eliminate most of the common mode noise on thermocouple wiring, composition difference of low-pass filter R1, R2, C2 placed in the front of the AD7793, can eliminate the thermocouple pin on possible pick up noise. R1, R2, C1 and C3 capacitance of common mode filter, can provide additional common mode filter. Reference voltage source and the use of ADI company AD3425, AD3425 is low cost, low power consumption, high precision reference voltage source, a & plusmn; 0. 1% of the initial accuracy.

when AD7793 configured for single polarity working mode, the analog input voltage signal to calculate the public for

type: AIN for analog input voltage; GAIN for the instrumentation amplifier GAIN; N for the ADC conversion digits; VREF as a benchmark voltage source.

apparently, reference voltage drift can affect the thermocouple signal acquisition voltage value. Conversion digits, the greater the least significant bit LSB is smaller, gathering the higher precision, the reference voltage of 2500 mV shall prevail, AD7793 least significant bit LSB is 38. 147μ V, if the AD converter replacement for 24 AD7792, least significant bit LSB is 0. 149μ V, precision improved 256 times.