Origin of the current signal

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It is clearly unreasonable to use voltage for transmitting analog signals over long distances, as there is a voltage drop due to losses in the wires. Some people claim that a high transmission impedance can be used to reduce the transmission current and thereby minimize the impact of this voltage drop, but this reduces the system’s resistance to interference. Converting the signals from sensitive devices into current signals for transmission can eliminate the voltage drop errors caused by the transmission lines, but what value is appropriate? Our twisted pair characteristic impedance is around 50 ohms, while that of wires with a width of 1 CM and a cross-sectional area of 0.2 square millimeters is around 300 ohms. Therefore, it is ideal to choose a load resistance in the range of 50–300 ohms. For the convenience of AD conversion, it is appropriate for the maximum signal level on the load resistor to be between 5–10 V. Taking all these factors into account, a load resistance of 250 ohms, along with a current of 20 mA and a load voltage drop of 5 V, represents a satisfactory combination. So why choose 4-20mA instead of 0-20mA? To reduce the complexity of wiring, using a 2-wire sensor is much simpler than using multiple wires. The 2 wires are used both for transmitting signals and for powering the sensor; therefore, designers draw 4 mA of current to power the sensor’s amplification circuit, which is how the 4-20 mA standard was established.