The general voltage in industry is 0... 5(10)V or current 0(4)... 20mA is used as an analog signal transmission method, and it is also a method often used by program control machines. So what is the difference between the transmission method of voltage and current, and when to choose what method, the following will be briefly introduced.
Voltage signal transmission, e.g. 0... 5 (10)V If an analog voltage signal is transmitted from the transmit point to the receive point via a long cable, the signal may be easily distorted. The reason is that the output impedance of the voltage signal passing through the transmitting circuit, the resistance of the cable, and the touch resistance constitute the voltage drop loss. The resulting transmission error is the sum of the input bias current of the receiving circuit multiplied by each of the above resistors.
If the input impedance of the signal receiving circuit is high-impedance, then the transmission errors caused by the above resistors are small enough that these resistors are negligible. The requirement that the signal receiving circuit has a high input impedance without increasing the cost of the signal sender and that the mentioned resistance is negligible. If an op amp OP is used as the input amplifier for the receiver, it should be considered that the input impedance of such amplifiers is generally less than <1M?.
In principle, high-impedance circuits, especially at the input of amplifier circuits, are susceptible to electromagnetic disturbances, which can cause obvious errors. Therefore, with voltage signal transmission, it is necessary to find a compromise solution between transmission error and the influence of electromagnetic disturbance.
Conclusion of Voltage Signal Transmission: If the electromagnetic disturbance is small or the transmission cable length is short, there is no doubt that a suitable receiving circuit can be used to transmit the voltage signal 0... 5 (10)V.
Current signal transmission e.g. 0(4).. 20mAIn environments with strong electromagnetic disturbances and long distances for transmission, people have preferred to use standard currents to transmit signals for many years. If a current source is used as a transmitting circuit, the current signal it provides is always the desired current and is independent of the resistance of the cable and the touch resistance. In other words, the transmission of the current signal is not affected by the hardware equipment. Contrary to the method of voltage signal transmission, because the low input impedance of the receiving circuit and the current source suspended to ground (the actual output impedance of the current source and the input impedance of the receiving circuit form a parallel loop), electromagnetic disturbance does not have a large impact on the transmission of the current signal.
Conclusion of current signal transmission: If the transmission interval must be very long, considering that there are electromagnetic disturbances, such as electric welding equipment and other signal transmitting equipment, then the method of current signal transmission is suitable for this situation (analog signal transmission). In fact, the current transmission methods that are often used are two-wire system and three-wire system. Here we will first discuss the two-wire method, also known as the current loop method.
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