Is there a "common voltage reference point" for all the circuits that connect together?

Question

Nowadays, the whole world is interconnected. We often say one device communicates with another device through the network. In this context, the 2 devices are viewed as 2 separate entities. But I think a device is no more than a bunch of circuits. And the communication can be expressed/carried out through voltage difference and current exchange.

In the case of voltage difference, I don't view devices communicating to each other as separate entities, instead I prefer to view them as connected circuits as a whole, i.e. they form a giant circuit. And the communications are just state changes within this giant circuit.

If my view is correct, I think there should be a common voltage reference point for this giant circuit. Does it exist? Where is it?

Answer 1

Yes, it's called ground, and it's not always a perfect reference. For example, ground loops may occur, wherein different devices see slightly different ground voltages--this can arise due to voltage drops resulting from ground currents, or directly from induced voltages in loops due to changing electromagnetic fields.

Not all devices use ground in the same way--for example, Ethernet uses small transformers at each end to send an AC signal without regard for matching ground voltages at each end. Isolated transceivers can use similar techniques to work with mismatched grounds, and optical signalling (optoisolators or fibre) have the same benefit.

Here's an example of the isolation and other signal conditioning from an Ethernet application note by Texas Instruments; note element T1 which includes transformers and chokes, and note further the high-voltage-rated series capacitor in the termination circuit, which keeps the Ethernet signals from being tied to the local ground.

Likewise, differential signalling is tolerant to some ground mismatches; while in many cases, the devices are referenced to the same ground, the differential signal ignores ground differences. Below, a signal is sent differentially, with the same common-mode noise (e.g. from noise on the ground line) seen on both lines at the receiver; the original data is faithfully reconstructed as long as the ground loop voltages are not so severe as to reach the limits of the receiver. If the ground loop is too severe, then you will need to fall back to isolated devices such as optoisolators, capacitive coupling, transformers, etc.

Answer 2

I'm going to join the people saying "no" here.

There are actually quite a few circuits that carefully isolate one part from another.

Transformers

From the planet Cybertron...oops, no, not them. The old standby is a transformer. A typical transformer has a core (typically thin sheets of iron, but sometimes other stuff like ferrite) with some wire wound around that's connected to one circuit, and another wire wound around it that's part of another circuit. The two "windings" (pieces of wire) are insulated from each other though.

The "primary" winding is driven by an incoming signal. It forms a magnetic field in the core. That magnetic field in the core then drives current in the secondary winding. So, we can transfer power across, but there's no electrical connection between the two at all. Transformers are routinely used both for transferring power, and transferring communication signals.

In a fair number of cases, a secondary winding will have a center tap that's often connected as the ground in the circuit powered by the transformer. The two "outer" wires from the secondary aren't connected directly to the primary, but they at least sort of correspond to the two inputs to the primary. The center tap, on the other hand, doesn't even correspond to any input.

Opto-isolators

When you're dealing solely with communication (not power), it's also pretty common to use optoisolators. An optoisolator is basically just an LED next to a phototransistor. An incoming signal turns the LED on and off. The phototransistor senses when the LED is on or off, and controls current flow based on it. The result is basically the same: we allow communication between two circuits, even though we isolate them, so there's no actual electrical connection between them at all.

Capacitors

Yet another form (so common many are likely to almost forget them) is the simple capacitor. A capacitor is simply two conductors separated by a (usually quite thin) insulator. An incoming signal causes a static charge on one plate, which causes a static charge on the plate next to it, so the output tracks (to some degree) with the signal on the input--but (again) from a viewpoint of sharing a common ground (or anything similar), the two are isolated from each other.

Summary

Although it is pretty common for circuits to have a common ground, it's also fairly common to completely or partially isolate circuits from each other in various ways that allow communication between them, without any common reference point or any possibility (short of a short in the circuit board, or something on that order) of actual current flow between the isolated sections.

Answer 3

The short answer:

Devices are often connected together via an Earth or ground line, forming your "giant circuit". The 19th century Telegraph and early 20th century Telephone were good examples. But use of such circuits is far from universal; they can have problems with interference, and on a large scale can be expensive and inconvenient.

Signals can be transmitted in other ways, such as via magnetic fields (in an isolating transformer), radio, optical couplers or fibres, etc. This is often done so as to avoid the need for a "giant circuit" and its attendant problems. Radio transmission was the earliest such communications solution and became widely known as "wireless". For example you probably have a smartphone, which is just a modern wireless device.

Another answer gives a deeper technical summary.

Answer 4

No aether

The problem is, you are using terms applicable to wired electrical circuits, to describe the entire milieu of electrical, radio, and other signaling. Then trying to armwave it out to the entire universe, as if radio waves, dirt and atmosphere could create a conductive groundplane. You're essentially arguing for an "Aether", a universal groundplane for all things.

That may work for a Bodhisattva, but it won't work for a Bodhisattva's porch light.

If there was a way to pull that off, commercial interests would have found it, as there'd be a lot of money there.

Except for Australia.

But if you crank up voltage enough and limit current, you can force earth to work as a return (albeit with some losses). That's what an SWER is. Naturally, leave it to Australia to be the exception to everything. It delivers power to some of its farthest reaching cattle stations and the like.

Answer 5

Only if you actually have a common circuit and then you get to choose your reference point.

The best reference is 'ground' but you can choose any you like even if it makes circuit analysis harder.

Separate circuits connected via RF or esp or light or whatever do not have any common reference globally for all of them, but could each have one locally by themselves.