The reason why current signals are more resistant to electric field coupling interference than voltage signals
Thread Content
The output type of analog sensors is generally either a current signal or a voltage signal. Current signals offer better resistance to interference and are more suitable for long-distance transmission, while the transmission circuits for voltage signals are less effective at resisting interference caused by electric field coupling. A current-mode signal is equivalent to a very large voltage source in series with a very large internal resistance. In this way, when the load impedance is much smaller than its internal resistance, the current is always equal to its “short-circuit current”. The main advantage of current-type signals is that as long as the resistance in the transmission lines and the contact resistance at the connections are not too high, and as long as their sum remains far smaller than the internal resistance of the signal source, it can be assumed that this does not affect the magnitude of the current received; it will still equal the \"short-circuit current\" of the signal source. In general, current-type sensors ensure that the error in the current value remains within a certain limit, as long as the total voltage drop across the load and the transmission line does not exceed a specific threshold. In practical applications, devices that receive current signals always prefer to have as low an input impedance as possible. In this way, in addition to the accuracy issues mentioned above, it also provides interference resistance. From the analysis above regarding the current signal at yunrun.com.cn/tech/1776.html, it can be seen that the current signal is not affected by voltage drops along the transmission lines, but it is sensitive to leakage currents. With leakage current present, the received signal naturally becomes inaccurate. However, leakage current is often related to voltage. With the receiver having a low input impedance, the voltage is naturally very low. The leakage current will not be very high either. Also, the common interference signals caused by electric field coupling are, in principle, similar to a signal source connected in series with a capacitor; this capacitor is a parasitic capacitor distributed in space, and its capacitance is very small, which means that the current of the electrical interference signal is also small. However, if our receiving circuit is receiving voltage signals rather than current signals, the input impedance is often very high (Note: A high input impedance when receiving voltage signals is necessary in order to minimize the impact of resistance in the transmission lines). As a result, when interference signals combined with distributed capacitance are applied to this input terminal, the voltage division resulting therefrom can be significant. Therefore, the transmission circuit for voltage signals is less effective at resisting electric field coupling interference compared to current signals.
Current signals are more resistant to interference compared to voltage signals, mainly because current-based signals function like a voltage source with high internal resistance. By controlling the current and maintaining a low input impedance at the receiving end, the voltage variations caused by resistances in the transmission lines have little impact on the signal. Furthermore, the voltage at the receiving end of the current signal is low, thereby reducing the leakage current and interference introduced by electric field coupling. In contrast, the receiver of a voltage signal typically has a high input impedance and is susceptible to interference caused by electric field coupling. .
When transmitting signals over telecommunications, the receiver uses a sampling resistor to generate a voltage drop, which is then amplified by an operational amplifier to meet the input requirements of the AD conversion chip, thereby converting it into a digital signal. If the current supplied is 20 mA, and a sampling resistor of 1 ohm is used, then a voltage drop of 20 milliohms will be generated. This voltage drop is then amplified by an operational amplifier and converted into a digital signal. To reduce the input impedance at the receiver, one can simply use a smaller sampling resistor. As for voltage signals, if the voltage is not high, they can be connected directly to the AD conversion chip; if the voltage is high, a resistor is needed to divide the voltage. In either case, the current flowing through the AD chip is very small, which results in a high impedance. Interference involves inductance and capacitance; the inductance is coupled between two leads, and it is the inductance that couples interference signals to both wires. I guess capacitors have a significant impact on signal lines, especially high-frequency signals. If it is RS232, which is not a differential signal, the impact of inductive coupling is significant. The voltage of the coupled interference signal is high, but its power is low, as energy is not transmitted directly to the signal line; instead, it is transferred through electromagnetic induction. As a result, the amount of energy involved is small, which means the induced current is extremely low, and its effect on the current signal is negligible. And in the case of a voltage signal, if there is no shielding layer and the wires are not twisted together, the interference voltage levels on the two wires cannot be identical; therefore, differential subtraction is not possible, and the impact is significant.