Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
The considerations for circuit board layout are several textbooks worth of electronic theory. Although a short answer here won't come close to everything you need to know, I'll touch on a few key concepts to get you thinking.
Analog and digital layout are related, but there are special considerations for each type of signal. RF is a special case of analog that is closer to high speed digital than it is to lower frequency analog.
A printed circuit board is actually a component in the circuit which is mounted on it. The traces have capacitance, self inductance, mutual inductance with nearby traces and structures, leakage currents, dielectric losses, etc.
Analog signals contain intelligence in amplitude, phase, and frequency. When routing these signals, the designer has to keep in mind all of the adjacent structures and signals that can influence the intelligence contained in the analog signal - i.e. voltage drops, propagation delay, capacitive coupling, induced signals, modulating currents, etc.
Digital signals contain intelligence in the pulse edges. The pulse edge is used to start and stop digital events. No intelligence is transmitted in the amplitude or pulse frequency. Anything that changes the shape, or position of the pulse edge will change the operation of the circuit. The edge of a signal can be considered to be a broadband collection of frequencies (read up on Fourier). The impedance of the circuit board trace becomes significant to the higher frequency components, impedance matching becomes significant to preventing reflections of the higher frequency edge components, the dielectric properties of the circuit board affect the propagation time of high frequency pulse edges, the rate of current change (di/dt) of fast edges can more easily induce high amplitude crosstalk in adjacent traces or structures than can analog signals, etc.
Because digital signals are so rich in frequency components, generally have higher voltages than circuit board analog signals on the same board, and have such fast rates of change at the edges, we try to keep them away from the analog signals. All of the properties of digital signals can distort the intelligence (amplitude and phase) in adjacent analog signals.
High current analog signals can also work against digital signals on the same board. An analog peak occuring at the same time as a pulse edge can cause the power supply to the logic producing the edge to dip, which causes the digital signal to rise to a lower 'on' state - that change in 'on' amplitude will be reflected as a shift in the slope of the pulse edge, which looks like a delay in the pulse edge - this is called 'pulse jitter' and causes timing problems in the digital circuitry.
Much more should, and could be said....
Like I said at the beginning - there is a lot of stuff to consider. If you are doing layout as a hobby, just jump in and do it. Most of the hobby type circuits don't use high speed digital clocks with fast edges, nor do they use sensitive mixed signal analog devices. You can learn by doing. Once you have chosen a circuit, do a Google search to see what others have done with the same type of circuit. You can also ask questions here about the best approach for specific designs.
There is no "one size fits all" cookbook for designing circuit boards. You have to know electronic theory, and how to use your knowledge. If you are going to design commercial circuit layouts, you need an electronics engineering education to do it right. At least start reading everything you can get your hands on. There is no substitute for curiosity, and a desire to learn.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
