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The analog circuit interface industry usually uses voltage 0-5(10)V or current 0(4)-20mA as the analog signal transmission method, and is also a method often used by the program control machine. Then what is the difference between the way voltage and current are transmitted, and when is the method used, which is briefly described below.
Voltage signal transmission such as 0-5(10)V
If an analog voltage signal is transmitted from the transmission point through a long cable to the receiving point, the signal can be easily distorted. The reason is that the voltage signal passes through the output impedance of the transmitting circuit, and the resistance of the cable and the contact resistance form a voltage drop loss. The resulting transmission error is the sum of the input bias current of the receiving circuit multiplied by the respective resistors described above. If the input impedance of the signal receiving circuit is high impedance, the transmission error caused by the above resistor is small enough, and these resistors are negligible. The requirement to not increase the cost of the signal sender and the mentioned resistance is negligible requires the signal receiving circuit to have a high input impedance. If the op amp OP is used as the input amplifier for the receiver, it is important to consider that the input impedance of such an amplifier is typically less than <1MΩ. In principle, high-impedance circuits, particularly at the input of the amplifier circuit, are susceptible to electromagnetic interference and can cause significant errors. Therefore, with voltage signal transmission, it is necessary to find a compromise between the transmission error and the influence of electromagnetic interference.
Conclusion of voltage signal transmission: If the electromagnetic interference is small or the transmission cable length is short, a suitable receiving circuit can be used to transmit the voltage signal 0-5(10)V.
Current signal transmission such as 0(4)-20mA
In environments with strong electromagnetic interference and the need to transmit longer distances, people prefer to use standard currents to transmit signals for many years.
If a current source is used as the transmitting circuit, the current signal it provides is always the desired current regardless of the resistance of the cable and the contact resistance. That is, the transmission of the current signal is not affected by the configuration of the hardware device. The opposite method of voltage signal transmission is that the electromagnetic input has no transmission to the current signal due to the low input impedance of the receiving circuit and the current source suspended to the ground (the actual output impedance of the current source forms a parallel circuit with the input impedance of the receiving circuit). Have a big impact.
Conclusion of current signal transmission: If electromagnetic interference, such as electric welding equipment and other signal transmitting equipment, is considered, the transmission distance must be long, then the method of current signal transmission is suitable for this case (analog signal transmission). In practice, current transmission methods are often used in two-wire and three-wire methods. Due to the importance of the two-wire system, the two-wire method, also called the current loop method, will be mainly discussed here.
Comprehensive characteristics of the current loop
Simple use: If the operating current of the signal transmitting circuit and other connected circuits remains constant, the operating current and signal current can be transmitted through the same cable. One only needs to use a load sampling resistor, and the voltage drop across the load resistor can be a useful signal. Of course, it should also be noted that the operating voltage should be high enough to meet the voltage drop required in the current loop.
Low cost: Compared with digital signal transmission, an AD conversion is required. Compared with a suitable driving circuit, a simple current loop method requires only one cable, one load resistor and one measuring voltmeter. Especially when the measurement accuracy is high, the difference between the cost of the two products is more obvious.
Error diagnosis: The advantage of 4-20mA current signal transmission is that it provides automatic error information in addition to long transmission distance and strong anti-interference ability. When a calibrated system outputs a zero signal (output current is 4 mA), if the received signal is greater than zero mA and less than 4 mA, then the system must be faulty. If the received current signal is zero, then the cable must be broken or there is a problem with the signal reception. If the current signal exceeds 20 mA, it means that there is a problem with the signal overload or signal reception at the input.
Long-distance transmission: The transmission distance is related to the driving capability of the transmitting signal terminal and the resistance of the cable and the measuring resistance (load resistance) at the receiving end. If a measuring instrument is also installed in the signal transmission cable, the load resistance should also take into account the input impedance of the measuring instrument and the input impedance of the monitoring recording instrument. These meters are often connected in the current loop and directly from the 4 mA operating power supply because of the low cost and no need for an external power supply. Therefore, the load capacity of the current source loop should be considered in the circuit design.
The simplest case of two-wire current signal transmission is an adjustable current source and resistor composed of the current signal sender and receiver (receiving signals from the load resistor), see Figure 1.
In Figure 1, the sender has an assumption that it should produce a desired signal current IOUT = 4-20 mA associated with the measured value. The resistor RL acts as the receiver and can measure the voltage drop VA above it or directly measure the ammeter in series with the circuit to get IOUT. In fact, the sender is often combined by many functional circuits. In the sensor field, the sender is often used as a signal measurement converter, which contains a sensor, a working power supply that supplies the sensor, and a current source (Figure 2). In addition to the measurement signal, the current source loop can also be used as an isolation amplifier for the output stage of PWM modulation pulse width signals, or simply as a signal source for voltage output.
Figure 2: Complete Current Signal Transmitter Circuit Typically the sensor signal or PWM modulation pulse width signal is varied from zero to a full-scale value (FS), then the voltage-controlled current source must be capable of generating a zero point of 4 mA and full scale. A current signal of 20 mA (difference of 16 mA). In industrial control, the control device (control room) often has a long distance from the measurement signal transmission circuit. If the power line is also used as a signal transmission line, the signal transmission can be performed with only two wires, and the cost will be reduced, and the circuit will be simpler and more reasonable. This is the so-called two-wire current transmission method.
Figure 3: Two-wire application circuit As shown in Figure 3, one provides a working power supply from the control device (control room) that can supply power to multiple transmission circuits simultaneously.
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