# Common vs. ground

I've been trained to use the word ground and the ground symbol to refer to the earth, the dirt on which we stand.

Circuit common is considered to be a separate concept, with the symbol .

My understanding is that "ground" means that the node can and should be tied to the earth. "Common" implies an arbitrary voltage reference with no connotation about its voltage relative to earth. One might have multiple commons in a schematic, but earth is earth.

Obviously there are cases where "ground" is used to mean what I defined above as "common". Would those cases be wrong? Does my use of terminology match standard usage?

The lowly NAND gate has a pin called GND and there's no reason to suppose it must be connected to: -

• Ground,
• Earth,
• Chassis,
• 0V or
• Common

I don't believe there is standard usage other than in protective earth points: -

• I'm not sure what you mean by "there's no reason to suppose it must be connected to...". Do you mean it's not clear which of those it's intended to be connected to, or it's not obvious that it needs to be connected at all? – kwc May 1 '14 at 17:26
• @kwc There is no overriding reason to connect to any "ground" or "common" circuit node. – Andy aka May 1 '14 at 19:04
• Maybe not at first glance if you think of the gates as abstract logical devices, but when you look at how they're actually implemented as transistors, doesn't it become clear that connections to ground and a positive voltage are necessary? (For example, en.wikipedia.org/wiki/NAND_gate#Implementations) – kwc May 1 '14 at 19:24
• @kwc - not at all. Consider if the positive terminal of the supply is grounded. There are plenty of circuits I've seen like this. – Andy aka May 1 '14 at 19:39
• Ok, I think I get what you're saying - there's no reason the node connected to the pin called GND actually needs to be a common or ground node? Do you have an example of a circuit like you described? – kwc May 1 '14 at 20:02

There are, in fact, two distinct concepts:

(1) the circuit reference node which has, by definition, a node voltage of 0V. All other node voltages are referenced to this node. It is the node that one connects the black lead of one's voltmeter when measuring a node voltage.

(2) an 'infinite' source (or sink) for electric charge with an unchanging potential.

It is, unfortunately, the case that both concepts go by the name "ground". Like you, I prefer the "common" for the first concept and "ground" (some use "earth" for an obvious reason) for the second.

The two symbols that you show are essentially synonymous. I have seen them used interchangeably. Sometimes in a given schematic, one symbol will be used for analog ground and the other for digital ground. It is up to the documentation or schematic notes to describe exactly what is meant, and how the grounds are ultimately connected together. They should certainly both always be used to describe the same nominal potential, which would be the zero potential of the circuit.

All this signs indicate a specific return path.

This is a return through the earth to reference power (lowest potential).

Used with true earth ground (NEC) to do absolute voltage measurements. In electronics we often use this as a generic symbol, adding only mess in measurements. Used for sensitive signals or power leakage.

The correct symbol for high current return (common) is this one

often called “chassis earth” and it is a simple thick wire or the metal chassis that the circuit is mounted. This point used for measurements as common. You can imagine a dual power supply.

Another type of return is the “floating return”

or the optional

This is mainly used as signal common and it is separated to analog and digital common then connected together to a single point ground.

Circuit grounding is simple at the face of it but gets involved when you look at details.

the symbol is the most generic and "common" one. The other one is just a variant of it. IEEE defines the latter as "signal ground" but in practise there are no fixed meanings, different companies and individual designers may use one or the other. What these are usually used for is if you've got different "grounds", you want to make it explicit what they're referering to just by looking at the schematic. You very well may have separate signal ground, what many people do is to simply name the ground net e.g. PGND, SGND.

Very often you have got "earth" i.e. chassis ground which may or may not be isolated from the circuit ground. That's where shielded connectors connect to. There are different schools of thought whether chassis ground and circuit ground should be coupled galvanically, via caps/ferrites or not at all. Chassis ground may connect to the mains protective earth (PE) depending on how your device is powered.

Mains and PE is often isolated from your circuitry so you don't get electrocuted in the lab and shorts connected directly to mains tend to have more spetacular outcomes than the same short via isolating AC/DC converter would.

Yes this is typical. ground just means a relatively low impedance common potential to absorb stray Electric fields or become a reference point such as the cetre tap or neutral of a 3 or 1 phase centre tap transformer.

Even earth ground can be many ohms relative to "Mother Earth" or deep ground rod in moist soil.

So whatever symbol we use, we mean for symbolic purposes to be treated as same potential voltage, although reality depends on current flow and actual quality of ground resistance.

So in the end it is a universal Symbol with standard meanings depending on domain of application.

Physics gives a third option: the inside of a conductive shell is an infinite source/sink for charges, same as an infinite conductive plane (or a sphere of dirt 8000mi dia.)

To any circuitry contained inside a conductive enclosure, the inner surface of the enclosure will act just like a ground-stake driven into the dirt.

This is caused by the "Faraday Ice-pail" effect, where any charges deposited on the inner surface of a conductive shell will migrate to the outer surface, and also distribute themselves so as to cause zero e-field within the shell (so they become undetectable from within.) The charges seemingly disappear, no matter how many mega-coulombs are dumped there.

Note that this only works if the entire circuit-assembly remains inside the shielded enclosure. If any wires extend outside, the enclosure no longer acts like a perfect ground: the "icepail" is ruined.