Thanks for respond. Reading the answers here also making some search on web, I am trying to get over confusions in my head.
But on some websites, it says in single ended wiring (signal is always references to EARTH ground) but I really don’t understand it. why they emphasize the earth. I think it can be earth grounded but without Earth connection just circuit ground is also be fine.
What do you think about this ? Thanks in advance (I hope I could explain my question well)

You need to consider WHAT single-ended wiring they are talking about. It is very likely they are talking about a particular application and then their claim is only applicable to THAT application.

What you need to learn isn't what a particular word means, you need to learn several concepts. That of the common reference point in a circuit, that of the chassis connection in a circuit, and that of an earth connection in a circuit. What are these three things? What is important about them? Why do we care? THEN you can start analyzing whatever circuit you are looking at and make much more reasonable decisions about how to determine what their use of various words means.

 

Circuit ground and earth ground is two different definitions, although they can also be connected in some cases, the confusion arises when the terms are incorrectly used and the wrong symbol(s) are used, especially when the earth symbol is commonly used for any power supply common, as it often is.
Earth grounded implies it is actually referenced to an actual earthed conductor.
Max..

Thanks for answer Max,
I’d like to ask one more and last question about this topic. Does ground loop happen only when the circuit is earth grounded ? I mean if circuit isn’t earth grounded , circuit ground (referance point) can also cause ground loop ?

 

Generally, in the arena I operate in at least, a ground loop is prevented by ensuring different parts of a system are grounded or connected to a common plane in order to prevent voltage levels or induced signal occurring between different parts of the reference plane.
In order to help reduce this occurring, a practice known as equi-potential bonding is carried out where all metallic components of a system are taken to the earth ground to reduce any randomly induced currents and/or EMI.
In industrial wiring that is earth grounded it is customary to take this common reference back to a central point called a Star Ground point.

In a circuit that is contained in a isolated piece of equipment, a ground loop can still occur where a potential difference occurs between common power reference points.
Max.

 

Thanks for answer Max,
I’d like to ask one more and last question about this topic. Does ground loop happen only when the circuit is earth grounded ? I mean if circuit isn’t earth grounded , circuit ground (referance point) can also cause ground loop ?

Ground loops not only don't require earth grounds, they don't require involvement of the circuit "ground" (reference point) at all (which, when all is said and done, is an arbitrarily chosen node).

When two nodes that are supposed to be at the same potential aren't (usually because they are, in theory, the same node), it can cause problems. Because this most commonly occurs with the node that we use as the reference, we lazily tack the word "ground" to it. Because the problem manifests itself because we have multiple taps onto that node, we call them "loops" because those multiple taps form physical loops. Those loops also form physical antennas that can radiate or pick up electromagnetic signals causing all kinds of problems.

The reason that ground loops so often involve the circuit common is that we naturally tend to pick as our common reference node the node to which the most connections are made. The more connections that are made to a node, the more chances there are for creating a loop that is significant. The other common places for ground loops are connections to the supplies. But they can appear on just about any node.

 

Ground loops not only don't require earth grounds, they don't require involvement of the circuit "ground" (reference point) at all (which, when all is said and done, is an arbitrarily chosen node).

.

I had not meant to imply that all ground loops occurred in earth grounded systems as per my last clarifying sentence.
I was making the point that in the field that I work in the ground loop concern occurs or can occur and cause unwanted effects in the earth ground wiring or body of the system.
Max.

 

I had not meant to imply that all ground loops occurred in earth grounded systems as per my last clarifying sentence.
I was making the point that in the field that I work in the ground loop concern occurs or can occur and cause unwanted effects in the earth ground wiring or body of the system.
Max.

I was purely replying to the TS's question. I hadn't even read your response at that point -- and I've got no problem with it.

In sensitive electronics there are competing grounding schemes. One the one hand we want a single-point (star) connection for some purposes (particularly the prevention of DC ground loops), but we want a multi-point connect (a ground "plane") for other purposes (particularly the prevention of high-frequency ground loops). These can be made pretty compatible by routing DC signals via a star topology and creating a multi-point ground plane via stitching capacitors. This also allows you to address inductive loops in signal traces that cross cuts in power planes because you can stitch the power planes together to form a single AC ground plane even though they are at different DC potentials. This let's you keep the high frequency image currents close to the signal trace which minimizes the inductive loop area.

 

What is a ground loop?

We often think of a power supply or battery supplying current. Remember, whatever current that leaves the battery must return to the battery.



Let us just look at the return path for this discussion. Let us think of this connection as a wire from A to B. The return current flows back from the load at B to the battery at A. Which of the two nodes, A or B, can we reliably use as a common point or 0V node in a circuit?

For current to flow, there must be a potential difference between point-B and point-A. Normally, we might treat either point-B or point-A as common or 0V reference point in our circuit. In some applications, this may be a safe assumption. In another application, this could be a detrimental assumption.

In essence, a ground loop occurs when current flows between two nodes when the application depends on there being zero current flowing between those two nodes.

This is where the concept of a "star" connection becomes important. If you want to establish a common reference point, you will connect all your common points to the same node, for example A.