PCB Design Guide – Schematic Capture


If you’ve completed your circuit design and know which pcb design software you want to use you’re ready for the third step – schematic capture.

Schematic Capture Basics

Schematic capture is the crux of a solid circuit board design. Whether you’ve inherited a pcb design, trying to design your own printed circuit board, or just trying to understand how to read a schematic, this section is for you. Here you’ll find information on many different aspects of schematic capture like the components of a schematic, what a good schematic looks like, and how to read and work with schematics.

In the electronic age we live schematic capture software has become readily available. Today you can download many different free pieces of schematic capture design software. Most of these tools are user friendly and allow hobbyists and seasoned engineers to begin capturing their schematics in minutes. However in order to become proficient there are many standards practices that should be followed. Following these standards will greatly reduce your design and debug time after you’ve had your circuit board manufactured.

What is Schematic Capture?
Schematic Capture Standard Techniques

Component Creation

Your pcb design software likely comes with several of the standard components already loaded in a library. You’ll want to use those components when available, but its inevitable that you’ll need to create your own custom library of components. You’ll need the manufactures datasheet for the component and begin the process of adding the necessary pins to the part. You’ll then be able to instantiate these components where needed to form your schematic.

Wiring up Components

To make all the components interconnected you’ll need to place wires between pins you want connected. This is a pretty straightforward process and will probably become second nature in no time. It should be mention to avoid 4-way ties at all costs. They only cause confusion.

For power and ground you can use universal symbols to make connections. This greatly reduces the amount of wires traveling across your schematic. In the netlist all pins found to be connected to the symbol will be connected together. It’s highly likely that you’ll want symbols for each of the planes in the printed circuit board. In other words, you’ll want a symbol for digital ground, analog ground, power ground, +5VDC, +12VDC, etc.

If you’ve got a large schematic you’re going to need to send wires across several sheets of schematics. There are generally “ports” to accomplish this task. These ports and can be inputs or outputs and will connect signals across several pages. Nicer pcb design software packages will also automatically tell you which page the signal can be found. This greatly reduces the amount of time it takes to examine a schematic during debug.

Design Rule Check

Any respectable pcb design software with schematic capture will have a schematic level design rule check. You should run this periodically for a couple reasons. You don’t want to get to the end of the schematic capture process to find that you’ve been doing something wrong and then have to do it all over again. It will also help you get familiar with the process and fix any minor issues along the way. The design rule check generally looks for any reference designators that are duplicated, inputs that are connected to inputs, etc. This tool can’t tell you if you hooked up your schematic correctly so double check your work!

Netlist

Once you’ve got a clean well organized schematic, its time to make a netlist.  The netlist is what the pcb design software uses to figured out what components it should instantiate and how those components are connected.  Generally speaking you can capture a schematic in Orcad Capture and then layout a board in Mentor PADS.  Orcad has become the industry standard but is having a hard time holding onto the pcb layout portion of their design suite.

Advanced Schematic Capture Articles

With electrons moving faster and faster across the traces, there are several circuit design techniques that must be followed in order to ensure your pcb works properly after fabrication and assembly. Here are a couple excellent resources to help ensure your design words as you expect.

Considering EMI / EMC During Schematic Capture

PCB Design Guide (Page 1)

PCB Design Guide (Page 3)