"Ground" and "Earth" are actually two different things, but can be at the same potential. In other words, the ground of your circuit does not have to be connected to earth ground, which is the potential of the return or ground of the mains in your service box on the house. This is always at Earth Ground because it is connected to a rod that is driven into the ground. There are many situations where your chassis ground or case ground on the instrument would not be at the same potential as earth. There are also instances where you want a different supply ground or circuit ground than the earth ground or the chassis ground. Thus terms such as chassis ground and common come into play. When these potentials are different, the schematic will use different symbols to denote zero volts, common, analog ground, digital ground, chassis ground, and earth ground. Using different symbols to sort these out keeps this from getting too confusing.
MP
