Schematic capture

- the stage in the PCB design process where the circuit or "schematic" is captured and entered into a computer.

With computer based PCB design and many more computer simulations being used in the design of electronics circuits, the schematic capture or schematic entry stage of an electronics circuit into a computer is now an integral part of the design process. Typically complete software suites are now used, and the schematic capture stage only needs to be undertaken once, and the information used not only for being able to produce a drawing of the circuit, but also data entry into the PCB design as well as circuit simulation, production of the Bill of Materials (BOM) and many other applications.

Schematic capture in the design process

The schematic capture part of the design process is today undertaken interactively. Prior to the schematic capture of the design, the initial high level design must be undertaken. Then in years gone by, breadboards of the circuit would be made up and made to work before committing to the schematic stage. Now with highly sophisticated circuit simulation software, the circuit is designed interactively during the schematic capture stage and the circuit simulated using software rather than building a hardware version of the circuit.

By using a computer based system for schematic capture, it is possible to enter very complicated circuits into a computer relatively quickly. It is also possible to undertake the design of the board and perform circuit simulations while the basic design is underway. In addition to this, many circuit capture systems provide a means by which the circuit revisions can be managed and configuration controlled properly. Where a circuit is being repeatedly updated, and there may be the possibility of several people working on different areas, this is of great importance.

Elements entered into a schematic have a shape associated with them for the schematic. In this way a shape designed for a particular part will be pre-drawn and appear on the circuit every time that particular type of part appears on the circuit. When using an end-to-end design suite, the full shape may also include the PCB outline, pads and the like. In this way the part number for that part defines all the elements of the part for the design.

Practical aspects of schematic capture

One of the big problems with computer based schematic capture systems now is that the circuits are often very large and they can become unreadable and difficult to follow. There is a trend to print the circuits out on A4 or letter sized paper, and each sheet may only have a few components.

Schematic error checking

It is very important to ensure that any schematic that has been captured is fully checked. While the simulation and other applications now available as part of an end-to-end design suite will trap and highlight many errors, some can still get through. Errors that creep through can be quite subtle. One that has been seen is where a particular node may be given slightly different names on different sheets. As the names are different they will not be connected by the computer. For example a node may be labelled "0v" on one sheet, but could appear as "gnd" on another. Accordingly it is very important to ensure that errors such as these do not creep through. Discipline in naming is essential.

Computer based schematic capture has greatly simplified the process of drawing circuit diagrams. Circuit schematics can be drawn as the circuit is designed, and managed in such a way that there is little room for error. While errors can occur, the level of errors has fallen dramatically with the introduction of circuit schematic capture software.

By Ian Poole

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