A \|/ Groundplane \|/ serves as the Return Path for All currents.
Like Bypassing and Decoupling, the Groundplane is a subject that is rarely covered in the classroom; but is emphasized in many Application Notes, and used in most real designs!

Again, the legacy is that many Design Engineers and Technicians ignore it at their peril!

A Groundplane serves as the return path for All currents, signal and power. 
It is desirable to not have different signals sharing the same ground return paths; the groundplane helps in that goal.
A continuous conductive planar having no discontinuities is the ideal groundplane.
A groundplane is a special and very important component of any circuit. In essence, it is the return path for all signals including the power distribution. The groundplane can be thought of as homogeneous for the DC power only. In all other situations it is strictly inhomogeneous. All this means is, that all grounds are not the same.
As various circuits use the ground plane for their signal and power return paths, currents--conducted and induced--are caused to flow throughout the ground plane, and potentially can affect any or all other circuits or signals, and can cause problems. 

One can start to understand the function and design of groundplanes if one does the following: 

1) Draw a map of all signals in a circuit, their inputs, outputs, paths, and their various connections to and from the groundplane; 

2) Then model the inductances, capacitances, parallel and series resistances while noting the power distribution paths and returns and their respective noise content; 

3) And don't forget all bypassing and decoupling devices and their contributions to the model; 

4) Since the groundplane is mostly inductive, note must be taken of any other inductances in proximity to the groundplane, such as shields, transformers, chokes, tuned circuits, etc., and their contributing fields at all relevant frequencies.

'' There are only two ways to model the groundplane in a complex signal environment: and nobody knows what they are ! ''

             Good Luck ! 

All Grounds are NOT the Same!
--Op Amp using Groundplane for 
--all Return Currents.
A Groundplane in Action!
ADC Layout, Functional Diagram
Power & Ground, and Signal Distribution 
Please notice that--schematically--every signal has its own ground return.
In an "Ideal" Ground Return system, every current has its own separate return path; and does Not Share return path copper!
DEMO:  Return Currents in a  Groundplane
The groundplane demo uses an audible square wave with fast rise and fall times that is commutated or keyed at a slow rate such that it is a Morse code "A" (dit, dah), and its complement is a Morse code "N" (dah, dit). 

Because A & N are complements of one another, if both signals were summed using equal weights, there would be a continuous tone. If either signal were stronger or weaker than the other, there would be a (faint) "A" or "N" with a continuous tone in the background.

Both signals are amplified and drive onto the groundplane at adjacent corners, forcing the return currents to travel through the groundplane--the shortest distance. See drawing

In the demo, a signal tracer (green probe) is moved across the groundplane sensing the return currents in the copper clad groundplane.  At either extreme the Morse code A or N is clearly heard, at mid point, the two signals are of equal levels and appear as a continuos tone. 

Hear__A & N    Hear__A & N
(Not synchronized to animation)
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Groundplanes & Powerplanes
in multilayer Printed Circuit Boards
Powerplane Separation
The use of multi-layer printed circuit boards allows the use of multiple ground planes, as well as buried (under the signal layers) power and ground ( Vcc and Gnd) layers. These layers are sandwiched together and act as a very efficient distributed bypass capacitor. A variation on this is to have the power and ground ( Vcc and Gnd) layers as the outer or intermediate layers, thus shielding the buried signal layers; or some combination thereof. See EMC
The less homogeneous these planes are, the more difficult it is to maintain the circuit's integrity. There can be a myriad of circuit parasitics (resonant tuned circuits, etc.), especially in the groundplane. 

As it turns out, if one could place a homogeneous plane (e.g., a shield, etc.) over the less homogeneous groundplane these resonant circuits could be made less detrimental. See the Resonant Groundplane demo below.

In the real world printed circuit board above, these parasitics can be reduced by the proper placement and layout of the powerplane(s). That is to say, the close proximity of the co-planars reduces the parasitic inductances[1] while raising the resonant frequencies, which lowers the overall groundplane impedances; also the shunt "R" diminishes the "Q".  --All of which is GOOD!

[1] Parallel inductors = less inductance

This is a simple example of what can be a very complex problem; i.e., a non-homogeneous groundplane (many apertures) with many different return currents from many different devices running at various rates; having rise and fall times from a few nanoseconds, to fractions of a nanosecond--all sharing the Groundplane as their return paths.

This can give rise to device oscillations and instabilities, generating NOISE and Crosstalk, etc. This effect is due to the imperfect "A.C. grounding of active devices, and the many parasitic reactances that inhabit the circuit & board topology. This complex environment could, I suppose, be analyzed and the "tuned circuits" identified... --you get the idea.

To make the point that the use of GOOD Groundplane Design is important, especially in a mixed signal environment (analog & digital), an experiment that I call a "resonant groundplane" was designed.

The animation below shows a 4" X 6" copper clad board was used, and a narrow (1/8") strip of the copper cladding was peeled almost the length of the board, such that, electrically it looked like the letter "U." At the edge of the board where the peeling began, a 0.33µfd capacitor was soldered, bridging the planes on both sides of the narrow GAP. Then both sides are driven (the capacitor is shunting) with a loosely coupled sweep (signal) generator and adjusted until the resonance is found. The display presentation is of a swept network having a definite resonance. Then a separate homogeneous groundplane is brought into close proximity, which causes the resonant frequency to increase, as well as, causing the "Q" of the resonance to diminish.

The animation is of a Resonant Groundplane where a second Groundplane (or Shield) is brought into proximity, and the effect on its resonate frequency.
The implication in all this is that a GOOD groundplane design should have as few APERTURES as is practicable. And where apertures are unavoidable, shielding or a secondary parasitic groundplane can ameliorate the effect.
Analog and Digital Groundplanes
Mixed Signals Layout
AD-9020 ADC 

More Groundplane Examples
Wire Wrap, Bypass capacitor on Groundplane
Frequency Domain
Time Domain
Note how the Bypassing becomes more effective as the ground conductor approaches a continuous plane. Groundplanes are often used in high frequency circuit boards for this reason.
Circuit Board Layout
 CAD Drawing
  Signal Input
Signal Output
Power Input
 Groundplane Side
  Shield Side
Note the single grounding (reference) point.

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