Why is a current signal preferred to a voltage signal for long analog transmission?

Question

Some sensors act like current sources, and I have seen it several times, especially for very long wires even at outdoors like wind vanes. 4-20 mA current loops are used instead of 0-10 V voltage for instance.

What can be the physical explanation for this? How is current more advantageous?

(I'm also wondering in terms of EMI interference whether a current loop signal is more immune and why.)

Please explain this concept by using circuit diagrams, voltage current sources with some components. How common mode interference is coupled in both cases, etc. and why a current loop is immune to noise.

EDIT:

After reading the answers, here is what I understand(click to see the simulation diagrams and corresponding plots):

I apply common mode Vcm interference in all scenarios.

In the first top figure a current source with 1Giga Ohm impedance is transmitted via an unbalanced/inbalanced cable and even the receiver is single ended the output is immune to noise. (1G Ohm makes the noise small, the lesser this Rcur the more the noise at receiver)

In the middle figure a voltage source is transmitted via an unbalanced cable and the receiver is single ended, the output is very noisy.

In the bottom figure a voltage source is transmitted via a balanced cable and the receiver is differential-ended, and common mode noise is eliminated.

Is my conclusion/simulation correct to represent this question?

Answer 1

Actually what matters for immunity against noise is the power that is needed to disturb the singal.

I.e. a current signal at an input with nearly zero impedance is just as bad as a voltage signal at an input with nearly infinite impedance.

What is needed is a receiver with non-zero as well as non-infinite impedance so that the signal involves some power.
I.e.

• if the information is coded as voltage, there still should be some current flowing into the receiver and
• if the information is coded as current, there still should be some voltage across the receiver.

So both cases are similar, but you just have decide whether is is better to code the signal as voltage or as current (another alternative would be coded as power). For measurement purposes voltage or current signals are most appropriate.

A good wire for a current signal just needs to ensure that no current is lost (or inserted), i.e. ideally no leakage, i.e. perfect isolation. This can be accomplished in practice quite well.

A good wire for a voltage signal needs to ensure that no voltage is lost, i.e. ideally no voltage drop, perfect conductance along the wire. Unless you are using a superconductor this is almost impossible to accomplish in practice.

^{simulate this circuit – Schematic created using CircuitLab}

In any case receiver resistance should be well above 0 and well below infinity.
It's easy to have the isolation resistance practically infinite.
It's practically impossible to have the series resistance 0.

Therefore if the signal has to be sent down some distance along a wire it is better to use a current signal than a voltage signal.

Answer 2

Current is great in that it is equal at all parts of a conductor. I.e. if you are pushing in 15 mA from one side, the other side is seeing 15 mA even if it is 200 m away. This is very easy to sense and makes data transmission reliable.

The same is not true for voltage. If your conductor has a high impedance and has electrical interference, then your input voltage signal will degrade and a valid voltage may not reach the other side.

The noise immunity comes from the fact that current loops are a low impedance system. See here why this matters: Why are high impedance circuits more sensitive to noise?

Answer 3

Current signalling has different advantages in different situations, so there are several different answers.

In the case of low frequency signalling.

A constant current source (sender) has a very high impedance (and a CV one has a very low impedance). So when you put fairly high series resistance in, it has no effect: the CC source is already super high, what effect is a few hundred/thousand extra ohms going to make? Likewise when you couple noise into the cable (C1,2) the high source R means that both wires go up and down together - it is common mode noise and has no effect on the current. Meanwhile the receive end has a low R. This damps down any capacitively coupled noise, and is robust.

A voltage system is the opposite. The source should have very low impedance. Series R is going to matter. The rx needs to be very high input impedance or you get a voltage divider. It will capacitively pickup noise, and will be prone to damage. Capacitively injected noise flows through RSource, and you get differential voltages at the receiver.

^{simulate this circuit – Schematic created using CircuitLab}

In the case of high frequency signalling (e.g video)

The current loop has basically constant voltage on both sides of the cable. Therefore capacitance across the cable does not pass any current, and does not have any effect. The signal is immune to cable C, and is immune to extra C added to protect from noise and emi. Much less power is used as C does not have to be driven.

Answer 4

As far as I'm concerned, these are the two main reasons for choosing current loops in several cases:

• You don't care about the length/resistance of your wires. You can change a 3m wire to a 50m one, changing its resistance, the signal will be the same (as long as the source can deliver enough voltage/power, of course).
• You can detect damage and failure. If you get 0mA, either you sensor or your wire is broken. With voltage loops is not that easy to figure out.

About EMI, it won't affect most of the times. EMI usually comes at (very) high frequencies, way faster than your signal changes, so you can filter it.

Also, it seems this is related to the old pneumatic controls systems, where 3-15psi range was used.

Answer 5

Something else to remember regarding analogue signals is the ability to then integrate the HART communication protocol. HART (Highway Addressable Remote Transmitter) is a digital signal which is overlaid on top of the analogue signal allowing for additional information to be sent via the same wiring. Most smart industrial instruments nowadays operate with HART capability. So the benefits are far greater than just voltage drop and EMI.

Answer 6

There are many aspects of current signal vs voltage signal. This answer is strictly about the noise immunity aspect.

There are several types of noise: conductive, inductive and capacitive coupled noises. I consider inductive coupled noise only (assuming conductive and capacitive noise make no difference at the moment).

Inductive coupled noise appears in the loop as a voltage V_n (Faraday's law). How does V_n affect the signal being sent?

In the case of current signal, for example, 10mA, the current source maintains the current through out the loop at 10mA, correct any current noise (=V_n/Z) picked up by the signal cable.

In the case of voltage signal, for example, 4V, the voltage source maintains 4V at the output terminals only. V_source= 4V = V_n + V_receiver . The voltage source doesn't 'see' directly and thus maintains V_receiver against the noise.