When routing a differential bus on a printed circuit board, what specific measures should be taken? Why?
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Alina
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Answer ( 1 )
As a rule, differential pairs are used for signal reproduction and reduction of common-mode noise. Differential transmitters and receivers can do this with lower voltage signals and better S / N ratios that can be seen with a clean cat eye pattern.
For low-frequency applications such as RS485 communication at 115 k baud or less, impedance matching is not a problem as the board is much shorter than the wavelengths involved in reflections. From 25 MHz, you probably want to adjust the impedance to keep the signals free from reflections.
Impedance matching means using uniform widths and an even distance from an unbroken ground plane and few vias or other discontinuities. Parallel conductor tracks with constant spacing are also required for differential impedance matching. Geometry is important.
There are various options for differential impedance control. Over and under tracks or side by side tracks. Ground (or energy) planes under or over and under. We use PCB impedance calculators, which are available at several points in the network and calculate the impedance of different geometries, which are then adapted to the source or terminating impedance used. Some of these techniques span up to 4 layers, so it can be layer-intensive if you have many other circuits (that need to be to other layers or far away) or multiple diff pairs like in differential buses.
Usually, the trace length match is not important unless you have buses and clocks that must match the runtime delays. In this case, you’re talking near 100 ps per inch. Good routers keep the trance length and can combine them with labyrinth sections.