A PCB schematic is a logical and visual representation of an electrical circuit. It is the first step of electronic product design. You must follow standard schematic guidelines such as precision net labeling, and symbol standardization to have a well-structured and error-free design.
Today, designers use numerous EDA tools for schematics generation, such as Altium Designer, Allegro, Pads, Kicad, Tinycad, Express PCB, and Zuken.
Schematic is one of the important aspects of electronic circuit design. A good schematic helps you conceptualize a well-structured circuit diagram clearly depicting the electrical connections between various electronic components and overall circuit functionality.
It should also be noted that a technically correct but crowded schematic is still a bad one, as it might confuse you. Schematics can be an extremely valuable troubleshooting tool during circuit board repair as it traces out the connections in the circuit.
To achieve a successful design, follow these standard schematic guidelines.
Most of the design tools offer different page sizes. Generally, the tools would select the page size as A4. However, it should be noted that various other page sizes are also available. You should select the size based on the size of their circuit design.
The logical blocks of the schematic should be separated by pages. The pages can be named using the letters A, B, C, and so on. By doing this, we can place the pages in alphabetical order. An example of such a naming convention is shown below.
Block diagrams and revision history are often ignored by most designers to save time. However, they can be very helpful for other designers trying to understand the schematic. Product-based organizations mandate all such protocols and regulations.
Though it is not a direct requirement, the tool must have some references. Hence, the grid system is followed. Having grids helps you reference the parts properly and make their connection. Circuit components and connections must always be on the grid; this helps in probing the nets during analysis.
The page title block is present in the footer of the schematic page. It is a good practice to fill in all the required details such as page size, update date, revision, document number, name/function of the circuit, and company disclaimer. An example of the title block is shown below.
A page title block in the schematic
You need to write the necessary comments concerning the circuitry. The notes can be written on either independent documents or schematic pages. Generally, notes are provided on a separate page for complex designs. Examples of notes could be jumper status, PCB layout constraints/guidelines, etc. A schematic with notes can be seen below.
Schematic with notes and comments
The revision history contains the changes that were made to the design. This document provides information such as the date and description of the changes made, the name of the author and the reviewer, and review comments, if any. Revision history is generally placed on the first or last page of the schematic. An example of schematic revision history is shown below.
Example of revision history of a schematic
The table of contents lists the topics present in the schematic document. This page helps you easily locate a specific module in a complex and large design. This can be skipped if the design is small and simple. An example of a ToC is given below.
Table of contents of a schematic document
The block diagram represents the different modules in the design and signal flow. This greatly helps the reviewer to understand the design for review purposes. Block diagrams may not be required for simpler designs but are primarily used in more intricate projects. An example of a schematic block diagram can be seen below.
Block diagram of a schematic
Implement auto-grouping of repetitive PCB design blocks if you have redundant blocks in your layout.
11 Chapters - 96 Pages - 90 Minute Read
If the design is complex and contains many modules, a hierarchical design is preferred. The hierarchical schematic clearly displays the signal flow from one module to another, as shown below. A detailed view of each module can be accessed by clicking on the respective module in the hierarchical schematic.
Hierarchical schematic design
The table below shows the names of the generally used electronic components and their corresponding reference designators used in any schematic. The designators are assigned as per the IEEE standard. It is recommended to name the components with its standard reference designators. Also, always use capital letters to designate the schematic symbols.
The schematic diagram consists of different types of components, such as active components, passive components, and connectors. Active components include transistors, diodes, logic gates, processor IC, FPGA, Op-amps, and so on.
Components like capacitors, inductors, and transformers are referred to as passive devices. Creating new symbols is not advisable unless the symbol for that component is not present in the standard library.
Resistors can be represented in two different ways, as shown below. You should take care to keep up the consistency in the symbols used.
Resistor symbols used in schematic design
The unit of the resistance is ohms and is represented by the symbol ‘Ω.’ Sometimes, the symbol ‘Ω’ can be replaced with the letter ‘E’. You should ensure that consistent unit representation is followed throughout the entire design. All required data about the components should be entered in the design tool. This makes it easier to create a BOM at the end of the design.
Capacitors have two terminals, one positive and one negative. Care should be taken to mark the polarity of these terminals. An error in the polarity of the capacitor terminal might lead to exploding. The figure below shows the capacitor symbols from IEEE standards.
Polarized and nonpolarized capacitors
You should also ensure that the pin numbers assigned to the symbols match the footprint layout.
A transistor is a three-terminal semiconductor device. The terminals are the base, collector, and emitter. You should always refer to the component datasheet while mapping the pins in the footprint layout to the schematic symbol.
Transistor symbol with pin numbers
When the symbols are created, it is important to enter a description of the component. This is very useful for future reference or when the part is obsolete and needs to be replaced. Having these details on the BOM improves readability. The two images below show filled description fields of a transistor symbol.
Symbol description of transistor
Symbol description of a component (Image credit: Altium)
It is very important to create the op-amp symbol per the IEEE standards. Many designers often draw the op-amp as per their convenience, which tends to lose readability. This may happen due to a lack of understanding and experience of CAD schematic tools.
Op-amp symbol
When you create a symbol, it is recommended to have all the input pins on the left and all the output pins on the right. Similarly, power and ground pins can be placed at the top and bottom, respectively. In the image shown, the input pins are 2 and 3, the output pin is 4, and the power and ground pins are 7 and 4, respectively.
You should be careful while flipping or changing the orientation of the symbol. When we do so, there is a good chance that positive and negative terminals will switch their positions.
Failure to adjust the wiring after flipping symbols can lead to misalignments between the schematic and physical connections, potentially causing incorrect functionality or circuit failure. Hence, care should be taken to cross-check each symbol with the manufacturer’s datasheet.
Complex devices such as FPGA, memory, and microprocessors are called heterogeneous components. These components have different types of pins in large numbers, such as data lines, inputs/outputs, address lines, control lines, and power lines.
To retain clarity and readability, you should create multiple components of a single package, such as UxA, UxB, UxC, and UxD.
Some pins on components may have multiple functions, and this flexibility is usually denoted on the symbol, accompanied by specific details provided outside the symbol. For instance, pin AA5 below is assigned various functions, such as GPIO.6, CLKOK, PWM, or T3.
Additionally, external notes at the connection point specify the current function of the pin; for instance, the label “GPIO6” outside the symbol indicates its present function.
An example of such a heterogeneous schematic symbol of a component is given below.
Heterogeneous schematic symbol of a component
The symbols of power and ground pins are shown below.
Power and ground symbols
It is always a good idea to represent voltages with a ‘+’ sign since there could be negative voltages present on the board. You should follow a standard and consistent convention to represent the voltage levels and their sections inside the silicon.
For example, +3.3V_IO, +3.3V_DG, +3.3V_AN +1.8V_Core, +1.2V_LVCore, +2.5_Vref etc.
Similarly, different types of grounds could be present on the board. The symbols are shown below.
