Why we need common ground for Analog and Digital?

Current flowing through wiring will develop voltage
drops that shift the supposed “ground” reference to something other
than the desired “0” point.

The problem is not just an analog ground versus digital ground problem.

 It can rear its
ugly head in almost any analog circuit, and can even in all-digital circuit. It
is, however, particularly prevalent in mixed analog, digital systems.Ground
wires serve two different functions. Sometimes they are used to carry currents.
At other times you want the ground to serve as a rock-solid reference point for
making precision measurements. This is especially true in A/D circuits with
more than 8 bits of desired resolution. The best way to handle the ground
problem is to have different ground systems that connect at only one point. The
precision analog REFERENCE ground should always be designed so that an absolute
minimum of current actually flows through it. In practice this is accomplished
by having all reference ground connections terminating at a single point.
Firstly, all MEASUREMENT ground connections should use
individual traces that converge at a single REFERENCE POINT. The idea is to not
allow any current flow through one reference trace to affect any other
reference trace. What you want to avoid is having one long trace that snakes
all over the board, with many different ground connections being made at
different points along this trace. The currents will add up, causing different
voltages to appear along the trace.
Secondly, all GROUND POWER supplied to the analog
section should use another ground system. This ground system should have a
single connection to the REFERENCE ground point. 
Thirdly, a COMMON ground connection to any major
DIGITAL GROUND system should also be made to the single REFERENCE ground point.
If properly constructed, the REFERENCE ground point becomes a nice rocksolid
point from which measurements can be made. It will be COMMON to all other
system grounds, but this common connection should NOT itself experience any
current flow. Or at least the current flow should be absolutely minimal. In
extremely noisy systems the analog section may be floated or  solated from
other sections. In this case data is communicated from one section to another
via opto-isolators, transformers, or using other isolation techniques.
Complete isolation is also desired in medical systems
where even small currents can do great damage to a patient. The tendency when
laying out a printed circuit board is to just run a ground trace around the
board and have all kinds of things attach themselves to this running ground.
But if you desire precision measurements one must have a precision reference
POINT to which ALL measurements are referenced.

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