This tutorial requires:
DesignSpark PCB V11.0.0Differential pairs are used in many areas of electronic design to reduce radiated noise and to reduce susceptibility to external noise. They are also used at high frequencies to match signal propagation delays through the two traces to accurately replicate the transmitted signal at the receiving point.
Note: Differential pairs is only available with a paid DesignSpark Engineer subscription.
At low frequencies differential pairs are often used for audio and analogue signals, the requirements on the tracks are reduced to just providing a closely spaced ‘send’ and ‘return’ track with minimal requirements on the track length matching or track spacing for impedance matching.
At higher frequencies for digital usage such as USB signals, the track length maximum and the track matching are more critical with requirements on the allowed ‘skew’ (track length difference) and the track spacing which controls the impedance being specified.
DSPCB Engineer provides the required tools to aid the routing process to achieve the requirements above and generate track length reports.
The following video illustrates the written steps below.
Simple differential pair.
Here we illustrate the settings for a simple differential pair and the routing process.
This example is for the simpler low frequency requirements and is also the first step for higher frequency applications.
A differential pair from a connector to an instrumentation amplifier.
The differential pair requires net names to indicate that they form a pair.
The net name requires a suffix _P and _N (or _p and _n) and will then be automatically designated as a differential pair.
The schematic now has the required details for continuation in the PCB.
Create a "Net Class" for the differential pair.
With the simple example in the PCB editor we now need to set up the differential pair as a Net Class. Use <Shift> + T and select the Net Classes tab and click the <ADD> button.
The new form that opens shows the many options that can be configured, these will be discussed later in “part2” where we cover a more advanced design, but here we only need to add a name for the Net Class.
With the net class added we need to toggle the option to make it a differential pair.
The nets used as the differential pairs can now be updated to use this new class.
Add the differential nets to the Net Class.
In the Design Technology select the "Nets" tab.
For each of the differential pair nets change the Class from "Signal" to our new Class "Diff Pair".
The differential pair is now ready to route on the PCB. Close the Design Technology.
Differential pair configuration.
Right-click on one of the differential pair nets and select to route as a pair.
The mouse pointer will now change and indicate that you are routing a differential pair.
You are nearly ready to start routing the differential pair.
Differential pair recommended settings.
For this simpler example, we recommend the following settings to simplify the task.
First while in differential pair mode as indicated by the mouse pointer ensure the following settings are ticked.
“Allow Pick Connection” is required to start the net selection.
“Route Connection At Pair Start” will start the differential track at the first pads on the differential pair.
A similar recommended setting from the menu bar, Settings – Preferences <PCB Tracks> is “Double Click to End Track”, and select “Complete Track”.
You are now set to produce a simple differential pair track.
Differential pair routing.
Double click on one of the differential pair nets and click where you want the main differential pair track to start and it will auto connect from the “pair start”.
Lay out each segment and click to place to change direction. Complete the track layout and when close to the target end pads “double click” to complete the tracks.
Press <ESC> to exit the differential pair routing mode.
Differential Pair Report.
From Output – Reports you can also produce a “Differential Pair” report which will provide the track lengths and differences (Skew).
Advanced differential pair routing.
Part 2 will cover routing for high-frequency designs where track length and skew are important parameters. Setting up a guard band from other tracks, showing the track spacing and displaying the track length and skew 'real time'.
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