Question: In an audio circuit, such as a guitar's electronics (the only thing I ever dabble with!), I have always thought (probably simplistically) of every point along a continuous earth path being essentially all at the same voltage (a 'zero volt' reference voltage).

However, I was wondering - if you extend an earth wire from some point along that path, is it possible for that in itself to introduce noise to the circuit by changing the '0V' of the earth reference? (Possibly I am wondering "What is the difference between a long earth wire and an antenna" - but I don't know enough about antennas to know if that's what I'm asking :)

Answer: 
Yes, long ground connections can pick up noise, which means the ground in one place is a different voltage from the ground at another place. For single-ended signals, this ground offset voltage usually appears as part of the signal.

There are two common strategies for avoiding this situation:

- Use differential signals. The actual signal is encoded as the difference between two signals that are driven oppositely from each other. Ground offset then looks like a common mode signal, which can be largely ignored by the receiver.

- Pay attention to how things are grounded. Make sure all the grounds are tied back to one place without other connections to elsewhere, like the local power outlet ground. This ground net is then connected to real ground in one place only. Also avoid having deliberate current flow thru any ground connection. There should be separate returns for power current. Never use ground as a power return.
0V is only 0V at the point of reference - it gradually degrades as you move away from that physical position. It may be nano volts difference up close but can rapidly become milli volts and, if sensitive input circuits have 0V connections that are not at the same point, the several milli volt difference can be an AC noise-signal and is quite often very annoying.

This is why PCBs use ground planes but, these are by no-means exempt from this problem. Other systems use star-point wiring to keep any 0V connections at the same physical point but these can suffer from magnetic pick up of AC.

It's a difficult nut to crack sometimes. Here's a nice picture of how a digital signal can create both ground bounce and power bounce: -

[![enter image description here](https://i.stack.imgur.com/LZoRm.jpg)](https://i.stack.imgur.com/LZoRm.jpg)

The fast rising edge on an output will inevitably charge up a PCB track's parasitic capacitance - this is seen as a small "bounce" on the power rail and associated distortion (side effect) on the outputted signal. When the edge falls there is a ground bounce. Now the signal is slightly corrupted by importantly, for this question, the ground plane and power plane have pulses of current injected into them and these can affect other circuits closeby.

Hears another example of where gaps in a ground plane can cause "bounce" because the return current of a signal has to "spread" out around the gap: -

[![enter image description here](https://i.stack.imgur.com/Q17zc.jpg)](https://i.stack.imgur.com/Q17zc.jpg)

Here's another idea of how mis-positioning functional parts can wreak havoc on sensitive analogue circuits: -

[![enter image description here](https://i.stack.imgur.com/CK4Of.png)](https://i.stack.imgur.com/CK4Of.png)

The clever thing to do here is avoid common 0V connections for things that can cause ground bounce to each other - this is a form of star-pointing i.e. separate power and ground connections from each individual circuit function only connect at a single "clean" pair of nodes (usually at the output of a voltage regulator or battery). 

[**This**](http://www.te.com/documentation/whitepapers/pdf/3jot_2.pdf) appears to be quite a useful document that explains the phenomena