Analog signals are very common throughout the automation industry and can transmit signals related to variable characteristics, not just on/off signals like discrete control. However, for many novice users and many technicians, the differences between different signal standards may cause confusion.
     Two types of analog signals
     The two types of analog signals are voltage and current. There are different digital ranges within each group, but the overall categories of voltage and current are present in various sensors.
     The problem with understanding analog signals usually lies in the way people understand the control system.
     When you start STEM or engineering education, you might begin the journey of programmable control using microprocessors (such as the common Arduino or Raspberry Pi platforms) before PLCS appear on the workbench. These microprocessors always use analog voltage input, usually within the range of 0-5 or 0-3.3 volts.
     Therefore, we embarked on our career to learn how to measure the variable voltage on a voltage divider, which became the standard process for connecting analog inputs.
     Then we arrive at the actual real-world workshop (or in the PLC I/O class, if we are lucky), and face the reality that a large number of analog signals are actually providing current. However, in all of our training sessions, constant voltage power supplies have been the topic of discussion, while constant current power supplies have hardly been discussed or even mentioned at all.

The advantages of analog current signals
     Once we enter the automated workshop, we will find a feature that dominates all the other features: reliability. It is completely unacceptable if the signal between the sensor and the controller is lost or changed. Even the differences in cost (usually very small) can be masked by the differences in output quality. In these two fields, electric current dominates.
     Signal loss detection
     One advantage is that it can ensure that the sensor signal reaches the controller. The common current standard range for industrial sensors is 4-20 mA. Therefore, even if the process variable is at a value, there is still a space below the threshold, but the current is still transmitted in the circuit. If the current drops to 0 mA, the control system will detect a wiring fault: built-in diagnostic tool!
      If the current output exceeds 20 mA, the analog input card often allows for a slight increase in the input sensing range, which can be detected as an excessively high value, while still keeping the current below the safe threshold for human interaction (many studies have shown that higher than 25 mA leads to serious and sometimes fatal respiratory muscle complications).

Signal quality maintenance
  Signal degradation may be more common than a completely disconnected signal. It is normal for signal lines to operate together with machines, which will apply unstable induced voltages (EMI effect) to the wires. When this happens, the controller does not detect the fault but detects the change of the process variable, and thus wrongly adjusts the output signal.
     When a constant current power supply is used, the voltage of the sensor will respond quickly when the current flowing through the wire increases or decreases due to external factors. The response speed is usually fast enough that the controller can filter out the short spikes between the cause and the correction.
     EMI is not only a problem; the size and length of the wires can also affect the resistance of the circuit. The analog constant current source can increase its own voltage to overcome the increase in resistance and continue to provide an appropriate current. To some extent, it is like a closed-loop control signal, increasing or decreasing the output as external factors affect the system.
     The advantages of analog voltage signals
     Although current has some strategic signal advantages, voltage remains deeply rooted for several reasons.

Equipment simplicity
  Previously, we discussed those small microprocessors that use voltage dividers to read changing signals, because they are cheap and easy to create. This might be illustrated in the potentiometer, a common industrial knob used to adjust motor speed, set point and other properties. Only three wires are needed to connect it to the analog controller, and no additional components are required. Converting this device into a constant current analog signal is both difficult and time-consuming.
     Most sensors (devices that convert physical properties into electrical signals) change the current or resistance passing through the circuit. If this is not convenient within the range of 4-20 mA, an amplifier and a voltage regulator need to be added. On the other hand, pairing this simple sensor with a single fixed resistor can create an ideal voltage divider ranging from 0 to 10 volts.

Easy to measure
  Although reliability may outweigh most other factors, in many cases it may not be a major problem. If the signal line is short and not affected by interference, we can choose a signal that is easy to troubleshoot and replace when a fault occurs.
      Voltage can be easily measured with a voltmeter because this connection is a high-impedance parallel connection between the instrument leads and the load lines. On the other hand, the current-carrying wires require extremely sensitive Hall effect non-contact sensors; otherwise, the circuit must be disconnected when connecting to the online ammeter. Obviously, disconnecting the circuit is not ideal for operation.

Which is better, voltage or current?
  Like many debates on industrial standards, I cannot offer an opinion on which one is superior to the other. The statement is that for any system, there can be huge advantages, so it is necessary to thoroughly understand how to install, troubleshoot and design around any system.
      Although they all exist, the 4-20 mA standard is more common for automated sensing devices. If your experience only involves voltage signals, it will be beneficial to invest time in studying the current signal standards. With the right design choices, you can design the best possible system to balance signal reliability and the ease of troubleshooting, thereby creating a flexible and future-oriented control system.