Circuit Layout Is Determined By The Requirements Of The Components Used...

All electric circuits require some form of power (electron flow) to operate. But they also have 'signals' to process. In order for the signals to be useful for processing there is usually a minimum 'signal-to-noise ratio' that is acceptable for the intended use. Noise also increases with frequency. This is where layout and component selection become important. With careful layout many critical problems can be easily avoided.

A Simple Understanding Of Impedance Goes A Long Way...

Circuits utilize components and understanding the hidden effects that wire, grounding and power distribution have on noise, noise rejection and better fidelity (signal to noise ratio) during layout makes for better circuits. Always.

For this analysis we are only going to consider frequencies below say 20 mhz. Way outside the range of the signals for this discussion but still important to consider for circuit grounding and power supply distribution. This is also a very comfortable range where Impedance is reliably predictable.

First Rule: Ground is a reference point. A major reference point. Any noise carried on the ground will surely be introduced into the signal path that references it (connects to). It is not uncommon in analog circuitry to have several ground 'nodes'. Each node representing a reference point for certain signals. All these nodes would then connect to a single node. The power supply ground node. We used to call this node Mecca but I'm a little uncomfortable with that term lately.

Second Rule: All wire is Inductive. Inductance is a property where the resistive component of a path increases with frequency. For a ground path this means that as the frequency increases the wires appear longer (increasing in impedance). This applies exactly the same to component leads and circuit-board traces. Another influence of wire is Induction. Induction is a magnetic coupling between wires. Again the higher the frequency the more signal, or noise, is induced between wires. I should note that magnetic inductance like this is also a function of parallelism. Parallel conductors will magnetically couple while perpendicular conductors will not. And... The amount of coupling is proportional to angle between the conductors. This is one of the reasons that wire is twisted.

Third Rule: Applies only to powered circuits. How individual components connect to various power forms is extremely important. For each component it is usually necessary to 'decouple' the power supply connections. Remember... Even the power supply paths exhibit increasing impedance with frequency. To account for this good designs will use capacitors as close to the power supply points to store and deliver the energy sometimes necessary when components change state or signal level rapidly causing spikes in energy needed at the power connections. The faster the transient current change the higher the effective impedance of the power supply path. What decoupling is intended to do is store additional energy locally at the device power node when needed effectively lowering the impedance that the component sees looking back toward the actual power supply. In other words... It is good layout practice to treat the power supply(s) as high-quality signal paths.

In Practice...

Arrange components so that the leads are as short as possible. Arrange components so the inter-connect paths use minimal lengths. Make sure the ground connections are as short as possible. When laying out components grounding concepts should take priority over signal lengths. Don't underestimate the power supplies ability to ruin an otherwise good layout.