High Speed Design Techniques: Schematic Considerations
With my apologies to Richard Rodgers and Oscar Hammerstein for borrowing their lyrics, “Let’s start at the very beginning, a very good place to start…” In this case, however, we are going to be looking at high frequency and high speed PCB design techniques instead of a Hollywood musical, and the beginning happens to be the schematic.
You may argue that the place to start would be on the ground plane itself. Board materials, layer stackup planning, or even component placement are all aspects of high speed design that need to be considered upfront. While all of that is true, there is a lot that should be initially done with the schematic so that it better represents the high speed circuitry (ADC) before the board goes to layout. Let’s discuss what these things are.
Traditional schematic creation
Typically a printed circuit board begins with a schematic. Over the course of time, the methodology has evolved from paper schematics to schematic driven netlist generators, to fully connected schematic and layout applications. But in each case the purpose of the schematic was the same, to graphically represent the circuitry that exists on the actual PCB.
Schematics are used during the assembly, debugging and testing of the PCB, and then again out in the field if the board needs any repairs. As a result, schematics are usually drawn with great attention to detail to help the people performing these tasks. Components, reference designators, and nets will all accurately depict the logic of a circuit. However, the way that schematics have been drawn in the past hasn’t always been as helpful during the layout of the board. This is because schematics that are drawn to help a technician find certain parts or to condense information into fewer sheets, will not necessarily depict the circuit flow that layout PCB designers need for optimum place and route.
A good schematic is the first step to a good board
Organizing the schematic for the circuitry flow of a high speed design
Having an organized circuit flow shown in your schematic is a great help for the layout of a high speed PCB design. Good organizational practices, such as defining circuit flows to always go in the same direction (usually left to right) and grouping of sensitive components, will help in the understanding of what order the circuits need to be laid out on the board. For a design that is not considered high speed, it isn’t as imperative to group components for circuit flow. Instead, design engineers might be more concerned with optimizing space on the schematic sheet. This is why it isn’t unusual to find components or gates across the page from the parts that they are connected to. Sometimes those extra parts end up on another sheet altogether in order to compact the design and reduce the total schematic sheet count.
In a high speed design, however, an accurate depiction of the circuitry flow is much more important than the total sheet count. When the PCB is being laid out, the PCB designers need to see which components are required to be grouped together. This is important for the measured trace length routing of differential pairs, clock lines, data groups, and other high speed nets.
Another important detail that needs to be shown on the schematic of a high speed design is the correct grouping of the parts and nets of a signal path. For example, let’s say that you have a signal path that starts from a source part, passes through a resistor, and then terminates at a different part. On a non-high speed design schematic this would be correctly shown as three parts with two different nets making up two different circuits. In high speed design techniques, however, it is still three parts with two different nets making up two different circuits, but both of these circuits are considered as one signal path. It is important that the schematic depicts this accurately so that the two circuits are grouped correctly as one signal path and signal path and signal integrity when laying out the PCB.
Additional high speed information on a schematic
High speed design schematics can also be made more helpful to the layout of the board by adding circuit groupings and keepout information. This will take the guesswork out of placement, while specific side and location instructions will guarantee that critical and sensitive parts are placed where they should be. Trace lengths, differential pairs, controlled characteristic impedance lines, and trace tuning instructions will help in the routing of the PCB. Mechanical information about unique parts on the board or the enclosure of the device can also be helpful.
Take the guesswork out of the design and communicate what you expect
A more detailed schematic is the first step to a better design
Whether you are doing your own PCB layout or you are sending it out for design, make sure that you’ve prepared your schematic with what it will need to be successfully laid out. Don’t expect others to know instinctively what you want, you need to communicate what you expect. Showing an organized circuit flow in your schematic and including necessary design information is the best way to make sure that the design that is ultimately laid out is the high speed design and signal that you expected it to be.