Differential Pair Routing in Practice: Skew, Coupling and Length
A differential pair is not two single ended traces that happen to be adjacent. It is a transmission line whose properties depend on the coupling between the two conductors, and the rules that follow from that are frequently misunderstood.
What a Differential Pair Is For
The pair carries a signal as the difference between two conductors, and the receiver rejects whatever is common to both. That rejection is what makes the pair immune to ground shift and to common mode noise.
The second reason is the return current. In a well coupled pair, the return current of one conductor flows in the other, so the pair does not depend on the reference plane for its return path to the same degree.
The third is emission. A pair with equal and opposite currents has a small external field, which reduces radiation compared with two independent traces. Our high speed design rules notes describe the geometry used.
Coupling and Impedance
The differential impedance of the pair depends on the impedance of each conductor to ground and on the mutual coupling between them. Increasing the coupling by moving the traces closer lowers the differential impedance.
The common mode impedance is what the pair presents to a signal that is identical on both conductors, and it is generally much higher than the differential impedance. This is why a discontinuity that affects both conductors equally has little effect, and one that affects them differently has a large one.
The trace widths are chosen to give the required differential impedance with the chosen spacing and the chosen dielectric. Changing the spacing to gain routing room changes the impedance, and the widths must be recalculated.
Length Matching and Skew
The two conductors must have the same electrical length, because any difference appears as a conversion of differential signal into common mode and as a reduction of the eye opening.
The tolerance for skew depends on the bit rate. A useful rule is that the skew should be a small fraction of the rise time, which for a fast interface means a small fraction of a millimetre.
The matching should be done by adjusting the lengths where the pair is uniform rather than by a large detour in one conductor. A detour also changes the coupling along its length, which introduces its own discontinuity. Our power integrity notes describe how the pair’s behaviour is evaluated.
The Importance of Continuity
The pair must maintain its geometry over its whole length. A section where the two traces separate to pass around an obstacle has a different impedance in that section, and the discontinuity produces a reflection.
Where the pair must pass a via or a component, a common approach is to separate the two traces slightly rather than to allow one to detour alone, since the symmetric separation preserves the electrical balance.
The pair should not be routed over a split in the reference plane, for the same reason as any high speed trace, and the return path for the common mode component is the plane. Our EMI immunity notes describe the return path requirement.
Vias and Layer Changes
A layer change introduces a discontinuity because the geometry of the pair changes and because the vias have their own impedance. The vias should be placed symmetrically with respect to the pair, with a return via beside each.
An asymmetric via pattern adds skew and mode conversion. Where a pair must change layers, the two vias should be the same length and the same distance from the pair, even if that requires a slightly longer route for one.
The stub of each via should be controlled, since the stubs add reflections that affect the pair. Where the pair runs at a high data rate, backdrilling or a blind via may be justified.
Termination and the Receiver
A differential pair is terminated with a resistor between the two conductors, whose value equals the differential impedance of the line. The termination should be close to the receiver.
Where the receiver has an internal termination, the external one is omitted and the internal one must match the line impedance. A mismatch produces a reflection that reduces the margin.
The common mode termination, where used, provides a path to ground for the common mode component. Without it, the common mode can build up and radiate. Our design release checklist notes where the termination requirement is recorded.
Verification
The verification is a time domain reflectometry measurement of the differential impedance along the pair, which shows the discontinuities as changes in the impedance trace.
A measurement of the mode conversion shows how much of the differential signal becomes common mode, which is the quantity that the length matching controls.
An eye diagram at the receiver shows the result of all the effects together, which is the measurement that matters for the product.
Process Control and Verification
On a design of this kind, differential pair is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
Process Control and Verification
On a design of this kind, differential pair is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
Process Control and Verification
On a design of this kind, differential pair is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
FAQ
Do the two traces of a pair have to be the same length? They must have the same electrical length, which is the same thing in a uniform dielectric. Where the dielectric differs, the lengths must differ correspondingly.
Does coupling matter if the pair is over a solid plane? It matters for the impedance and therefore for the termination value, and it matters for the mode conversion when the geometry is disturbed.
What does gopcb provide for differential pairs? We provide trace geometry for the target differential impedance, impedance and mode conversion measurements on coupons, return via placement beside layer changes, and eye diagram measurements on the operating link where the customer requires them.
