4-20 mA vs. 0-10 V Monitor/Control Signals


Thread Starter

Charlie Hart

I am designing a piece of production automation equipment and trying to decide between the use of 4-20mA or 0-10Vdc monitor/control signals. I come from a lab environment and have always felt that the 0-10Vdc signal will inherently have better accuracy, resolution, and long term stability over 4-20 mA circuits. However, the consensus from the plant floor always seems to call for 4-20 mA circuits in order to minimize transmission errors. Resolution and accuracy is key to this project. I am looking for comments and/or information sources on this subject.
If you're starting from a voltage signal, converting to current then back to voltage, I think it's necessary but sufficient to select the current transmitter and voltage-dropping resistors to meet the required specifications for accuracy, resolution, and long term stability. I don't think there's anything inherently inferior in a current signal per se.


Ken Irving

Steve Myres, PE

I vote for 4-20 in most instances, particularly noise immunity is critical. The higher power level of the signal should inherently improve signal to noise ratio, and I don't think 4-20 transmitters are automatically any less accurate.
4-20ma signals are generally passed through a known resistance at the receiving end of the signal and the actual input to the electronics is voltage anyway. Just make sure that the shunt resistors are high precision type (like 0.1%) and
stable with time and temperature (whether you will be providing them or they are internal to your monitoring /controlling device). Also make sure you observe good analog wiring practices and I'm sure you'll be fine.
One consideration, between many other (i.e. electromagnetic field pollution), that you must have is the distance between the field variable sensor to the data adquisition device. If the data adquisition is near enough to process
variable probably you can use a (field variable)->(voltage) transducer. If the process has a high spatial distribution (there exist many process varibles in a big area) you must consider to use a distributed data adquisition system. The
signal flow may look like this:

(filed variable)->(voltage)->(local data adquisition unit)->(Communication Module)->(Field Network)->(Master Concentrator Unit i.e. PC)

You can use a Data Adquisition System with a Comunication Procesor that put through all signals into a industrial field bus like Profibus DP or Modbus between other.

With this option you can put de Data Adquisition System, with field bus communication capabilities, near as you can of the process variables transducer and then you can carrie the field bus cable hundreds of meters (this depend of the transmission media) without affect the signal integrity. In other words you can have a good signal as your transducer give at almost any distant off the process.

There also exist sensors that have embedded a networks capabilities allowing you connect them together. In this sensors networks you can access any measurement device individualy in order to get signals levels or for configuration.

Many thing like plant layout or hard industrial enviroments may let you think that put Data adquisition units near to the process field variables is almost impossible but the most of manufactures had developed many solution to almost
any situation.

Luis Rios
[email protected]

Terence B. Creevan

Well, one argument in favor of 4-20mA is that all components on the loop will see the same amount of current. Not necessarily true for a voltage signal. Voltage can be affected by resistance in your wire. However, I have used both with no observable difference.

[email protected]
With a 4-20ma signal you have 16ma measure a 0-100% signal, with a 0-10volt signal you have 10 volt measure a 0-100% signal. If you are sending the voltage signal any distance at all the voltage
drop in the wire degrades the 0-10volt signal.
The 4-20ma can be sent 2000ft over twisted pair.
The current signal has two advantages.

1. Line loss goes away
2. You can more accurately calibrate a signal which is positive rather than one which switches polarity.

Another potential problem comes in when you place multiple devices in parallel on the voltage signal. You may get some cross talk or interference. Series on a current signal will be less prone to this.


Jaroslaw Klak

Many measuring instruments can be supplied directly from 4..20mA current loop. Consider simplier electrical circuit, less wires in cable,
sufficient resolution and accuracy for most application.
4-20 mA provides the "live" zero, for broken wire detection. If signal < 4 mA, open circuit is implied. Controllers can be programmed to
recognize this situation.

Al Boake P.E.

George Condur

If you want to eliminate many problem note this advantages of 4-20mA:
1. a lost connection can be detected
2. reduced noise
3. more sensors with this option on the market

Because I can't trust in the electricians team, I prefer to use 4-20mA in my projects. They usualy do some small "inventions" that could compromise
measurements. Think that, other months, the customer install a power cable near the
measurement wires. I had this experience before.

Johan Bengtsson

The fact that you don't have a live zero of the 0-10V signal will on many input cards mean a higher resolution.

Why is it like that?
In most cases you put your current signal thru a resitor and measure the voltage drop, a typical approach is to use a 500ohm resistor and thereby get 2-10V. Most input cards (where you do like this) are designed to input either 0-10V or (worse in this case) -10 - 10V.

input signal card type used range
0 - 10V 0 - 10V 100%
4 - 20mA 0 - 10V 80%
0 - 10V -10 - 10V 50%
4 - 20mA -10 - 10V 40%

On some cards it can be even worse in reality so I would suggest checking it.

This means if accuracy is important and your input card don't have that many bits resolution this might be important to you. If it isn't and there are no other real reasons for either I would have used 4-20mA.

/Johan Bengtsson

Do you need education in the area of automation?
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
If 0-10V is going to a high-impedance input, why would there be losses(drops) over distance? Voltage drop is IXR.

If HIGH-impedance input, current is negligible, and drops/losses should be too. Yes, current is always the same in a series circuit, but that doesn't make it immune to losses due to conductor and connection resistances...... unless that is seen and accounted for at setup/calibration time. But connection losses can build up later over time (oxidation, dust, film, contamination).

Seems to me 0-10V going to a high impedance input would be the winner. Am I all wet here?
If the transmitting instrument is actually controlling current, it WILL be immune to those factors within the amount of voltage the loops has to work with. IOW, a dirty connection increases the loop resistance, thus tending to cause a decrease in current. Then the transmitter will then drive the output harder till it sees the loop current reach the desired value. The only thing that can cause a problem for a current loop (if actually current regulated) is if that loop resistance reaches the limit imposed by the transmitter or the system voltage, or if there is a parasitic PARALLEL current path around the load (again the transmitter senses current and has no way to know anything is wrong). Accidental parallel paths are much less common than added series loop resistance.
It seems that most people prefer 4-20 mA, I do too however if you are just using signals locally like on a skid there's no reason not to use 0 - 10 Volt.

Think about how you are going to power the instruments 2 wire 4-20 or 4 wire Voltage.

Are all instruments you need available in 0-10V? I doubt you would find industrial quality pressure transmitters for instance.

Can you connect all the negatives together supplied from one common power supply or will some outputs need signal isolators to avoid ground loops?

You might find the cost of using Voltage against current drives the selection.

Gerald Beaudoin

I agree with Johan's post in that you had better check the number of bits in the decoding of the signal. This will ultimately decide the maximum resolution possible. Then of course there is the separate issue of accuracy to also be considered. Beware that it is possible to have greater resolution than accuracy....so in that case all those digits after the decimal can be very misleading.

Gerald Beaudoin