What is a Circuit Schematic?By ZM Peterson • Aug 26, 2019
We don’t normally go back to basics around here, but many new designers can benefit from some advice on the transition from systems design to circuit diagram and, finally, a real PCB layout. If you’re wondering what is a circuit schematic (also called simply a schematic), it’s important to distinguish this from a block diagram and circuit diagram.
As part of your design process, you’ll need to start with a block diagram, circuit schematic, and eventually a PCB layout. Your design software has a major role to play here as it can help ease the transition between circuit schematic and PCB layout (more on that in an upcoming article). However, if you don’t get your block diagram and circuit schematic correct, you won’t have a working device and you won’t be able to easily transition to the PCB layout phase.
What is a Circuit Schematic?
A circuit schematic shows a component-level view of your electronic system. Components can appear as individual blocks in a circuit with inputs and outputs, or as the typical component symbols you would see in a circuit diagram. Complex devices like logic gates are shown as blocks to eliminate the need to show every single transistor and the passives in the component. This makes your circuit schematic a very convenient way to show connections between passive components, ICs, and other components like connectors.
Circuit Schematic vs. Circuit Diagram
If you remember your Electronics 101 class in college, then you likely remember being exposed to circuit diagrams at some point. A circuit diagram is different from a circuit schematic in that it shows a more granular view of how fundamental circuit elements (e.g., transistors, resistors, etc.) are combined to form an IC. In contrast, your circuit schematic will bunch a more complex circuit diagram for an IC into a single box with inputs and outputs.
A circuit schematic for a PCB links back to component models and graphical footprints for different components, whereas a circuit diagram only incorporates the fundamental circuit elements to show functionality. This is important for both simulation and layout. Within simulations, a circuit schematic will reference the individual circuit elements in a given component. This information will be used to solve the voltage and current throughout a circuit using Kirchoff’s laws and Ohm’s law; this is the basic idea behind a circuit analysis package like SPICE.
Circuit schematic for an equivalent transmission line model in Altium Designer 19
You can think of a circuit schematic as the blueprint for a new building. This fundamental document provides a guide to laying out your device on a PCB. Once you have finished schematic capture, you're ready to translate your schematic into an actual PCB layout. This process brings your CAD models directly into your board layout so that you can start routing connections between your components.
Circuit Schematic vs. Block Diagram
Sometimes, a circuit schematic and a block diagram are used interchangeably, but they are not intended to communicate the same information. A block diagram is really where your PCB design begins. The block diagram shows you a functional view of how the system works. When we say "functional view," we’re simply referring to the functions that manipulate data and analog signals in the system. Each portion of the system is labelled in terms of the function it performs. The goal is to focus less on components and more on how the system works. Note that a block diagram can certainly make references to components while still focusing on showing the functionality in the system.
Your circuit schematic can now be used to break each function into its components. For each functional block, you’ll need to construct a circuit that provides the desired functionality using a circuit schematic. For very complex devices with multiple block diagrams, you’ll want to use hierarchical schematic design. In this design methodology, each functional block is organized into its own schematic, and a parent-level schematic defines the connections between each functional block. This defines a parent-child-sibling relationship between each schematic.
Block diagram that shows some functions provided by specific components.
As you delve into designing a circuit schematic, you’re advised to use a PCB design program that includes schematic capture features and allows you to apply a SPICE simulation directly to your circuit schematic. Altium Designer, Upverter, Cadence OrCAD, Mentor PADS, and Zuken CADSTAR are all excellent PCB design packages that will provide this functionality, but they differ in their functionality in many other areas. We’ll look deeper at the difference between some of these packages in a later article, or you can find plenty of comparisons online.