Analog input...single-ended mode or differential mode 1

[RJC3PO]

Hello,
I have a question concerning analog input cards and their wiring. Specifically, if inputs are wired as “single-ended” or “differential”.

As I understand it, when using differential mode one is able to avoid potential (one never knows ahead of time) problems associated with external electrical noise from interfering with a signal input. This is because the 4-20mA signal (for example) and common are wired directly to 2 analog inputs and only the signal itself is sensed. All other noise is reduced due to the fact that this is typically wired with twisted pair cable and if external noise is added to the signal it's effect is effectively canceled out because both the signal and the common will experience it equally (common mode rejection).

In a single ended signal, the common is typically the reference plane (ground) and if both the transmitter and the input are not solidly (grounded) connected together or if noise exists on the ground plane, the difference will be "felt" by the input.

As I understand it:

Differential mode
Pros:
Noise reduction
Allows transmitter and input to be supplied from 2 independent sources without a common bond (assuming you terminate the shield on only one end)

Cons:
May cause ground loops if the shield is terminated on both ends or if the transmitter signal common is not truly floating.
Doubles wire required for the signal
Reduces analog input count by ½


Single ended mode
Pros:
Reduced wiring cost
No reduction in analog input count

Cons:
Increased noise susceptibility

When and why would you use differential as opposed to single ended, and when single ended as opposed to differential?
Does differential or single-ended mode change the signal resolution in the PLC/DCS?
Are 4-20mA signals inherently less susceptible to noise?
Is there a cable length advantage to single-ended signals?
I realize some devices are just inherently differential and some single ended but, if I had a choice of either, which would be best and why?
Thanks for the help!

 

[roydm]

RJC3PO
That's a well thought out question
I think the main advantage of differential is it gives you the option of using a non ground based signal i.e instead of 1-5 V it could be 7-11V.
I don't believe there is any cable advantage for single ended.
Roy

 

[itsmoked]

[blue]SEE ###[/blue]

Differential mode
Pros:
Noise reduction
[blue]### Only common mode noise reduction. The differential noise remains.[/blue]
Allows transmitter and input to be supplied from 2 independent sources without a common bond (assuming you terminate the shield on only one end)
[blue]### Not necessarily true a grounding difference can exist between the two locations and this difference can be more than the common mode range of the differential inputs. This can raise havoc with the readings.[/blue]

Cons:
May cause ground loops if the shield is terminated on both ends or if the transmitter signal common is not truly floating.
[blue]### Not so much ground loops as what I described above.[/blue]
Doubles wire required for the signal.
[blue]### Negatory. All your signals should be in twisted and shielded pairs. You DO NOT use single wires in single-ended setups or you will have many vexing issues.[/blue]
Reduces analog input count by ½.
[blue]### Nope.[/blue]


Single ended mode
Pros:
Reduced wiring cost
[blue]### Nope.[/blue]
No reduction in analog input count
[blue]### Often true. An Ain module can often have twice as many inputs with single ended setups. This is the whole point. They cost about half as much per point. Many, nay, most systems are single ended for this reason.[/blue]

Cons:
Increased noise susceptibility.
[blue]###Only increased common mode noise susceptibility![/blue]

When and why would you use differential as opposed to single ended, and when single ended as opposed to differential?
[blue]###You would use single ended unless you are making longer runs through areas of noise. Or if the system is laid out badly and has power conductors in close proximity to control signals. Other reasons would be if sensors have very small signal outputs.[/blue]


Does differential or single-ended mode change the signal resolution in the PLC/DCS?
[blue]### Not typically.[/blue]

Are 4-20mA signals inherently less susceptible to noise?
[blue]### Only voltage induced noise not magnetic induced current noise. Some here actually believe it isn't any better and they provide a good argument. At best it is a marginal improvement. Where it excels is in negating the cabling loss on long marginally sized wire runs. It works well in that case. Also it can reduce the wiring dramatically with respect to field transmitters.[/blue]

Is there a cable length advantage to single-ended signals?
[blue]### Nope.[/blue]
I realize some devices are just inherently differential and some single ended but, if I had a choice of either, which would be best and why?
[blue]### That my friend is a hard one to answer. As normally, as I mentioned, you would use single ended unless specific sensors need it or the installation reeks of noise issues.[/blue]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[PLCMentor]

I agree with itsmoked (nice name by the way) with everything except the point on reducing I/O count. I think there are some cards that allow a mixing of single ended and differential, but I would suggest that you should choose one or the other to avoid future problems as points are added to the system. So in my mind - yes - differential does cut the channel count in half.

He makes a good point on ground potential and multiple power sources, but you can bring in 2 power sources if you are careful. In your design, you may need to look at isolated input cards which will not have the single ended option, but will allow better isolation between channel. Single ended is great and the obivous choice if you have all loop powered devices from a single supply.

Russell White, P.E.
Automation Technologies, Inc.

http://www.AutomationNC.com

Automation Help

http://www.PLCMentor.com

 

[djs]

One big, main reason often trouted for differential inputs is to prevent "ground loops" or where the signal source and the signal reciever have the same ground. In some cases parts of the signal current may not flow through the sensing element, but rather through some of the other wiring. It has been my experience in these cases, even using differential inputs may not be enough. Whenever I encounter grounded signal sources, I use signal isolators. This is often cheaper than using differential inputs.