Single ended vs Differential

learner

Lifetime Supporting Member
Join Date
Apr 2007
Location
British Columbia
Posts
81
Ok, here's what's going on with me. As silly as this might sound, I just have a hard time fully understand the difference between Sngle ended and differential wiring.

Strange enough, theoratically, I can teach this to another person but I don't undrestand it fully. I have searched the web and the forum. Ya, I got something out of it but it's not perfect.

Anyways, I believe the confusion comes from all diffrent interpretation of COMMON, GROUNG, SIGNAL GROUND, SIGNAL COMMON, EARTH GROUND, SHARED COMMON. I just wnat to see if someone can just explaine it or guide me to the right direction once and forever I put this to sleep. Your example could be simply around a simple analog input module like 1756 IF8 or whatever. Just please be specific about the COMMON, and GROUND.

Thank you so much

Always a learner
 
First, establish in your mind the output signal of the PLC. 0-10V is relative to the negative side of the DC power supply. This is your zero reference. Forget the term Ground - it gets defined separately. Just call it Common for now.

Single ended uses one OpAmp. Differential uses 2. With single, we just measure the positive lead. The negative end is not measured. We assume it is at zero volts. Same potential as the negative end of the DC supply. The Common. With this format, all negative ends can be tied together. Signal common.

With differential, we measure both leads relative to Common. We need 2 OpAmps. The second OpAmp also measures the negative end of the signal, relative to Common. Our output is the difference between the output of the 2 OpAmps.

The Earth is an infinite source of electrons. It is the true zero reference for every discussion of electricity. The incoming AC is referenced to Earth. The AC neutral is tied to a rod driven into the Earth. In Electron theory, all Electrons come from the Earth.
RF energy, noise, etc. is all relative to Earth. We call this Ground. In the US, if the machine is tied to Earth, we call it Grounded. In the UK, it's called Earthed. The most accurate term for Ground is Earth Ground.

That's the true definition of Ground, but most use it loosely to define the zero reference. That's because in most cases. the frame of the machine, enclosure, and negative lead of the DC supply are all tied to Earth. In this configuration, Ground is also Common.

When your talking about the 0-10 volt output of the PLC, where are you connecting the negative lead of your voltmeter? That can be Earth, Ground, Common, Signal common. These terms can only be accurately defined by looking at the current system under discussion. Consider the zero side of signals, zero side of DC power supply, zero side of the PLC. Are these all tied together, or do we have different zero references?

The literal definition of each term is always relative to Earth. Ground doesn't have to be tied to Earth. In your car, the metal chassis is the source of electrons. The metal chassis is ground. The negative lead is also tied to the frame, making Common and Ground the same.

There is only two references to consider. Earth is true zero. When you discuss a voltage, it is relative to where you put the black lead of your meter. The difference between these two is the source of confusion.
 
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