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Sensor interface circuit of current output applications - Sensors, Technology - HC net electrical in

By: dream zhang

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Many in the SCM applications should use Sensor To the MCU can not directly measure the signal into a single chip can handle the electrical analog signals, such as Pressure Sensor , Temperature Sensor , Flow sensors. Most of the early sensor for the voltage output, will measure the signal is converted to 0-5V voltage output, through the analog-digital conversion circuitry for converting the digital signal processor reads the control. However, long-distance transmission needs in the signal or interference with the larger network environment, occasions, the use of sensor voltage output is restricted, exposed the disadvantages of anti-interference ability, but the current output sensor for its high anti- interference has been widely used.

Voltage output pressure sensor interference ability, and sometimes also the output of the DC voltage superimposed with AC components, the SCM produces incorrect diagnosis, control errors so serious that it damage the unit.

Current output sensors are commonly used in the output range of 0 ~ 20mA, and 4 ~ 20mA two, the minimum sensor output current and maximum current, representing the sensors are calibrated value of the minimum and maximum rated output.

Such as Pressure to 0 ~ 35Mpa the range of the output of pressure sensor described in an example. The sensor output 0 ~ 20mA 0mA current pressure values corresponding to 0MPa, the sensor output 4 ~ 20mA 4mA current value corresponding to 0MPa pressure, two types of sensors corresponds to 35MPa pressure 20mA current value.

0 ~ 20mA for sensor output in the circuit design, we simply select the appropriate step-down resistor, through the A / D converter directly to the resistance of the voltage can be converted into digital signals, circuit debugging and data-processing simple.

For 4 ~ 20mA output sensors, circuit debugging and data processing are relatively cumbersome. However, this sensor when the sensor line barrier, it can be detected through the normal range of current to determine whether the circuit fails, the use of more common.

4 ~ 20mA sensor's general approach
As 4 ~ 20mA 4mA sensor output when the voltage at the sampling resistor is not equal to 0, directly through the analog-digital conversion circuitry after the digital conversion is not 0, MCU can not directly use, through the formula is too complicated. So the general approach is through the hardware circuit will 4mA sampling resistor the voltage drop to eliminate, to conduct A / D conversion.

Generated by the pressure sensor 4 ~ 20mA current loop current and 13VG formed to some extent in the sampling resistor voltage drop, and the voltage input to Amplifiers LM358 3 feet. Resistor divider circuit is used to IC LM358 to 2 feet to produce a fixed voltage value, for the 4mA offset current sampling resistor voltage drop generated. So when the pressure sensor is 4mA minimum when, LM358 3-pin and 2 pin voltage is the basic difference is 0V. Connected with LM358 adjustable voltage amplifier resistor composition, the pressure sensor current sampling resistor in the voltage value amplified by LM358 output of 1 foot to digital conversion circuits for single chip CPU Read by look-up table or other data processing methods to pressure sensor 4-20mA current pressure value on the screen in the form displayed. [Popular Keywords]: Sensor   Technology Comment Large   In   Small

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