Anatomy of a Schematic

Circuit boards are works of art that take skill and creativity to design. But before a design can be turned into a circuit board layout, you need to create some basic drawings that define electrical connections in your design. These basic drawings are your schematics. These electrical drawings define all the connections that will be physically represented in the PCB layout. They serve many other uses too once the PCB is fabricated and assembled.

Before getting into all the possible uses of schematics, we need to look at how they are created and what the different CAD objects in schematics represent. In this lesson, we’ll cover some of the basic elements that appear in a schematic and explain their basic meaning. We’ll also look at how to structure a schematic so that you can create easy-to-read electrical drawings.

Important Elements in Schematics

Very basic boards may only have enough circuitry to fit into a single schematic, while other boards could require dozens of schematic sheets to show all the possible circuits used to build a PCBA. No matter how many sheets are used to show the circuits in your PCB, all sheets have some common elements that are defined in industry standards. This standardization helps ensure that you can read the same schematics in any ECAD application, and the schematics will always have a consistent meaning.

To see how schematics work and really see all the important portions of a schematic, let’s look at an example from the MiniPC project in Altium Designer. This schematic shows a circuit for a 10-pin connector. This schematic and circuit are very simple, but the schematic contains many of the elements you’ll find in many other example schematics.

Sometimes, there will be a lot of blank space in a schematic, while other schematics can be very dense and complex. ECAD software allows you to use multiple schematics to build your device, so don’t feel obligated to put every single circuit into the same schematic sheet.

At the top of the schematic we have a title. The title should briefly explain what the circuit does. In this case, the connector makes a specific type of connection, called a JTAG connection. The use of JTAG is an advanced topic and we won’t discuss it in this course.

In the bottom-right area of the screen, we have a title block. The “Title” entry can be the same as the title placed at the top of the schematic, but this is not always the case. Some of the other important information that is standard is the company logo, company address, page size, number, and revision. The number is the same as a page number; in other words, this schematic is page #9 in a set of 46 schematics. The revision entry tells you the current iteration of the design and is equivalent to a version number.

If you’re familiar with title blocks used in MCAD programs like AutoCAD, this probably isn’t new. The main difference between electrical schematics and drawings you create in other CAD programs is the lack of units or other notes in or around the title block. The other important information doesn’t need to be dimensioned and is instead placed around the schematics in an easy-to-read manner.

Now let’s look at the connector circuit in this schematic. The image below shows a zoomed-in view of this circuit:

Some of the items in the above schematic were introduced in the last unit, and they can be found in your glossary. We want to look at these in a bit more detail to see what each of these items means in your design. However, we first need to understand an important CAD object that defines the connections shown in the above schematic. This connection between different objects in a schematic is called a “net”.

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