Stripline PCB Differential Impedance
Calculate the differential impedance of a coupled stripline PCB trace pair.
PCB stripline refers to a type of trace/transmission line routed in the inner layers of a PCB. It is enclosed by a single material — usually the PCB substrate — and is most commonly found in multilayer PCB designs where the signal trace has ground planes both above and below it. This arrangement keeps high-frequency signals inside the PCB, leading to fewer emissions and shielding against incoming signals.
This calculator computes the differential impedance (e.g. D+ & D− from USB) between the positive and negative trace of the transmission line. The differential impedance is slightly less than twice the single-ended impedance — the closer the two traces, the smaller the differential impedance. Calculating differential impedance is a two-step process:
- Calculate the single-ended impedance (Characteristic Impedance)
- Input the characteristic impedance, space between traces, and trace height to determine the differential impedance
Understanding Stripline Differential Impedance
Key Concepts
| Concept | Description |
|---|---|
| Odd Impedance () | Impedance measured with reference to the ground plane when testing one differential trace — differential signals must be driven with opposing polarity; |
| Differential Impedance () | Impedance between the two differential traces — affected by trace width, trace thickness, separation between ground planes, and relative permittivity (DK) |
| Asymmetric Stripline | A stripline where the distance from the trace to the top and bottom planes differs — commonly found in PCBs and advantageous to model due to its frequent presence in designs |
Calculator Types
| Calculator | Description |
|---|---|
| Differential Microstrip Impedance | Finds differential microstrip line impedance — considers substrate dielectric constant, trace width, and trace thickness |
| Differential Stripline Impedance | Finds differential stripline impedance — considers relative permittivity (DK), trace width, trace thickness, and separation between ground planes |
Important Considerations
| Factor | Description |
|---|---|
| Etch Factor | Etching produces a trapezoidal-shaped trace rather than a rectangular one, altering the actual impedance — often not considered by online calculators |
| Propagation Delay | Most online calculators either omit propagation delay or estimate it using methods known to be inaccurate |
Conclusion
Stripline differential impedance plays a crucial role in high-speed digital systems and is impacted by several parameters including relative permittivity (DK), trace width, thickness, and distance between ground planes. For accurate design and simulation, it is imperative to use precise calculators and account for variables like the propagation delay and etch factor.
Formula
where:
- = Differential Impedance (Ω)
- = Characteristic Impedance (Ω)
- = Space Between Traces (mm)
- = Height of Trace (mm)
Inputs
Single-ended characteristic impedance of each trace in ohms
Edge-to-edge spacing between the two traces in millimetres
Height of the dielectric substrate in millimetres