What Differential Impedance Actually Is
Differential characteristic impedance describes the relationship between differential voltage and current for a propagating mode on a transmission structure. It is not the DC resistance measured with a multimeter. Geometry, conductor and dielectric properties, coupling and nearby reference conductors all matter.
The target and tolerance come from the controlling protocol or component specification for the exact generation and channel. Do not confuse characteristic impedance with receiver input impedance or the reference impedance used to normalize S-parameters.
Use a Source Matrix, Not a 90/100 Shortcut
| Decision field | Question | Controlling evidence |
|---|---|---|
| Protocol identity | Which interface and revision does the source/sink implement? | Protocol specification and endpoint data sheets |
| Quantity | Is the value differential characteristic impedance, common-mode impedance, input impedance, or S-parameter reference? | Definition in the named test section |
| Location | Does the limit apply to PCB, connector, cable, fixture, or complete channel? | Compliance topology and reference planes |
| Tolerance | What range and frequency/time-domain conditions apply? | Revision-specific electrical limits |
| Measurement | TDR, VNA/S-parameters, fixture characterization, or system compliance? | Prescribed method and calibration/de-embedding procedure |
| Coverage | Qualification sample, process monitor, or release test? | Customer validation and quality plan |
The USB-IF publishes the USB 2.0 specification and related documents; MIPI publishes a versioned D-PHY overview; VESA publishes versioned eDP material. Consult the relevant revision instead of extending a number from one generation or topology to every implementation.
Why 10Ω Is Worth Fighting Over
An impedance discontinuity can reflect energy. Its system impact depends on magnitude, electrical length, location, transition shape, spectral content and the receiver's available margin. A nominal ten-ohm difference does not predict pass or fail without that context.
Map the complete path before assigning a cable requirement:
source package → PCB launch → board receptacle → cable plug → cable → cable plug → board/panel receptacle → sink
The connector transitions and fixtures may dominate a local discontinuity even when the cable's uniform section meets its target.
How a Cable Hits Its Target Impedance
Differential impedance is influenced by conductor geometry and spacing, dielectric properties, coupling, shield or reference-conductor geometry and transitions into the termination. The approved cable and connector specifications should define the permissible construction; assembly processing must preserve pair geometry and shield/ground treatment.
TDR and VNA measurements answer different questions. TDR is useful for locating time-domain impedance changes. Frequency-domain S-parameters can describe insertion loss, return loss and crosstalk under defined reference conditions. Neither method is meaningful as a release claim until fixtures, reference planes, calibration/de-embedding, limits and coverage are agreed.
The One Rule for Selection
- Identify the exact source, sink, protocol revision, lane mode and data rate.
- Copy the target, tolerance and test definition from the controlling source with its revision.
- Mark the compliance topology and reference planes across PCB, connectors and cable.
- Select a connector/cable construction whose manufacturer data supports the application.
- Define fixture, calibration or de-embedding, measurement outputs and acceptance limits.
- Separate qualification, process monitoring and release-test coverage in the validation plan.
Related Pages
- RF and Impedance Testing
- LVDS Cable Assemblies
- eDP Cable Assemblies
- Further reading: What Is High-Speed Impedance Control
- Capability boundary: Electrical Test Planning
- Process controls: Build-to-Print Manufacturing
