Terminations and Differential Pairs

Choose where to terminate a line and preserve a fast differential link.

Why terminate a line

A transition travels along a trace with a characteristic impedance. If the impedance at the end differs, some energy is reflected. The signal may overshoot, ring, or cross the receiver threshold several times.

A termination makes the impedance seen by the line closer to the intended value. It is not chosen at random: type, value, and location depend on driver, line, receiver, and topology.

Two common placements

• Series near the driver: the resistor completes the driver output impedance. It limits the launched edge and often suits a point-to-point link.
• Parallel near the receiver: the load absorbs the arriving wave. Some arrangements draw continuous current.

Some FPGAs provide programmable termination or output impedance. This saves components but must still match the I/O bank and board.

Differential pairs

A pair carries opposite signals and the receiver uses their difference. To preserve this behavior:

• route both traces together;
• keep geometry and reference as similar as possible;
• limit length mismatch;
• avoid unnecessary branches and layer changes;
• place vias symmetrically;
• preserve the intended differential impedance.

A differential pair still needs a physical reference and return path. It is not immune to plane gaps or discontinuities.

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