The world of grounding can be pretty confusing, but proper grounding is essential to reduce EMI and improve EMC (See also EMI and EMC). A bad grounding scheme in either the PCB layout or in connecting devices or circuits can cause lots of undesired problems that are pretty difficult to investigate and solve afterward. While designing, you have to keep all the good practices regarding ground returns in mind in order to minimize noise (See also Noise), hum, and other undesired visitors that might be knocking on your door later.
A ground is in fact a reference connection with a zero potential (0V), but that is only theory. In practice, from the moment that a circuit is powered and currents start running everywhere, the ground potential will not be 0V anymore. Return currents going into a ground that doesn't have a very low resistance (for DC currents) or a very low inductance (for AC currents) will lift the ground potential up, so it is not 0V anymore.
When the voltage drops over ground connections becomes big enough, so DC currents or high frequency currents get trouble to return to ground properly, this causes all kinds of nasty problems like noise, ground loops and radiation.
When high frequency currents encounter inductance, they get radiated as electromagnetic radiation by this inductance. All the current that gets radiated as electromagnetic radiation doesn't return to the source, causing an unbalance between signal current and return current. This unbalance looks like a part of the current is running in the ground return opposite to the return current and in the same direction as the signal current. That is why they are called common mode currents.
So common mode currents are the result of inductance in the ground return, causing electromagnetic radiation, thus EMI/EMC issues.
Common mode currents are particularly harmful for signal integrity because they introduce noise that reduces the signal-to-noise ratio.
In schematic capture software, you find a variety of ground and earth symbols, which can be pretty confusing.
But let's not make things more difficult than necessary:
Protective earth/protective ground/equipment ground/earth ground:
Earth ground generally is connected to the (green/yellow) conductor in your mains distribution, that is connected to earth potential. This earth ground can be used for safety when a mains powered device has a metal enclosure. When the metal enclosure due to a problem would get connected to the live conductor of the mains power, fault current will flow directly to earth. This will prevent that the fault current can flow through a human when touching the enclosure.
Chassis ground:
Typically, the chassis of a device, having a conducting chassis, is connected to earth ground for safety reasons.
Reference ground/Signal ground/ ground return:
Typically, this is connected to the ground of the power supply. It can further be connected to chassis ground to use the chassis as a shield.
The ground reference is the return path for all kinds of signal currents, thus is also the signal ground.
Analog ground and digital ground:
In mixed mode systems, where analog and digital circuitry are shared, it is important to keep the analog ground plane separate from the noisy digital ground plane. This prevents that the noisy return currents of the digital part run through and affects the analog part. The analog ground plane should be kept as clean as possible. Only at one single point, the analog ground should be connected to the digital ground:
either at one point close to the mixed mode chip itself, while the reference ground typically is connected to the digital ground.
or both individual to the power supply reference ground for systems with multiple mixed mode chips.
This is f.e. important when using AD converters, integrated load cell amplifiers with an analog front end and a digital control unit.
Note that the power supply filter/bypass capacitors of the digital part of a mixed mode chip should be connected to the ANALOG ground to keep the noise down. The digital ground plane should be underneath the digital power plane. The same for the analog ground plane. This keeps both parts separated and avoids cross-current-contamination.
A ground loop is caused by millivolt level voltage differences between 2 grounding points. The voltage seems too low to even remotely cause any problems. But because of the very low impedance of the wiring involved, a low voltage can cause circulating currents of hundreds of milliamperes.
Put in 100 Ohm resistor
or using differential signals
or plug all instruments or audio equipment using the same wall socket