Layout-is the integration of all of the subjects, Plus a little ART
As with Bypassing, Decoupling, Groundplane, Shielding, Layout[PCB] is a subject that is mentioned in the classroom, but rarely taught. It is often described as " of those things that is done by Draftsmen." 

The truth is, not done properly, it can ruin the best paper design!

WHAT IS A Layout?
 Layout [PCB] is the creation of a physical realization of the circuit, utilizing all of the covered subjects; with the primary attention to Signal Flow. 
WHY USE A Layout?
In order that the circuit work as intended, there must be optimized interconnection of the circuit elements; while observing all pertinent rules. 

In addition, the proper layout, insures economical manufacturing and high reliability.

HOW TO DO A Layout?
Input, output, parts placement, signal flow and power distribution, all must be considered; with attention to interference and noise within the circuit layout. Device rules, such as bypassing, decoupling, shielding, the proper use of groundplanes, and last but not least, EMC, must ALL be observed.
Layout of a Printed Circuit Board (PCB) is the creation of a physical realization of the circuit.

The first step in this process is to determine the board size, type or standard, connections, etc.; this is often dictated by the application, e.g., PC adapter card, etc.

There are other constraints such as end use (industrial, medical, military, commercial, etc.); environmental; product life; and last but not least, COSTS.

Once these parameters are identified, there is a finite amount of flexibility remaining for the designer such as the number of layers, device selection, function integration, and LAYOUT.

Input/Output connections, Parts Placement, interconnections, Routing, signal flow, Power Distribution, all must be considered; with attention to interference and noise within the circuit layout. 

In addition, Device Rules, such as bypassing, decoupling, shielding, the proper use of groundplanes, and last but not least, EMC considerations, must ALL be observed.

During Layout, remember: in an "Ideal" Ground Return system, every current has its own separate return path; and does Not Share return path copper!

Of course, in the real world this is not always possible; however, one stands a much greater chance of a successful design keeping this in mind.

ADC Layout, Functional Diagram
Power & Ground, and Signal Distribution 
Incorporating Bypassing & Decoupling with Proper Ground Returns;
Please notice that--schematically--every signal has its own ground return.
Physical Layout
Analog & Digital
Physical Layout of the Analog Devices, Inc., AD-9020 Video ADC
Groundplanes & Powerplanes
in multilayer Printed Circuit Boards
Multi-layer printed circuit board
Powerplane Separation
Currents in either power plane can couple NOISE to the other; the coupling mechanism is mostly magnetic.
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
"Folded" Layout
Notice single grounding point
In laying out a board, it is important that Signal Flow of the circuit be the dominant consideration. Place the input connector as far away from the output as possible. Likewise, place the power entry to the board away from the input, and nearer the output. 

In the case of mixed signals, segregate Analog from Digital.

Neatness counts in writing, not in layouts:  Keep runs as short as possible.

If you must have long runs, don't parallel the same lines close together; if possible swap relative line positions to reduce crosstalk.

Separate Analog & Digital Groundplanes with single point unified grounding (COM)
Analog and digital groundplanes DO NOT SHARE return currents.
Buses are sometimes required to span long distances across a board, which can lead to Crosstalk. This can be ameliorated by allowing adequate spacing between traces ( see fig. B). However, there can be instances where this either doesn't work or there is not enough board area available; in which case interleaving ground returns may be required. (see fig. C & D) 

Bus Layout
A) Simple 16 bit bus, close spacing; B) Wide Spacing; C) Interleaved Ground traces, 1 ground per 2 signal; D) 1 ground per signal
I/O Termination
Notice that the shield, which is isolated above the cabinet, is returned directly to the device input common pin, not just to "ground." 

The Shottkey diodes protect against severe "Ground Loops."

Example of Coaxial Cable Termination

Circuit Board CAD Layout
 CAD Drawing
  Signal Input
Signal Output
Power Input
Shield Side
Note the single grounding (reference) point.
Computer Aided Design (CAD) package:
  1) Schematic Capture
  2) Part Selection
  3) Part Placement 
  4) Auto Routing
  5) Simulation

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