PCB and Transmission Lines
Model class: Standard approximation
Source-termination designer
Calculate a first-pass series source-termination resistor from line impedance and driver output resistance.
Interactive engine
Start with the stated conditions.
Values stay in this browser. Choose a representative scenario, then calculate deliberately.
Example ready
Calculate to inspect the result.
The result will identify the direct answer, assumptions, and any warning that changes the next decision.
How much does each input matter?
Each row holds every other input fixed at its current value and moves one input by ±10%, using the same model as the main result. It identifies which input is the binding lever, not a full tolerance stack.
Assumptions to check
- The entered source-termination values represent the stated operating condition.
- This standard approximation is evaluated in the declared lumped or first-pass model.
- A source-termination calculation is not a component qualification or safety approval.
What this source-termination calculation establishes
Calculate a first-pass series source-termination resistor from line impedance and driver output resistance. The useful result is the stated electrical quantity and the decision it supports, not an unstated claim about a finished product. This engine keeps the governing relationship visible so an input, unit, condition, or model boundary can be reviewed before a value becomes a component or layout choice.
First-pass series resistor is line impedance minus driver output resistance, so the combined source impedance matches the line. Treat the number as a first-pass result for the declared operating point. When a source, load, temperature, frequency, waveform, component tolerance, or measurement condition changes, repeat the calculation at the relevant corner rather than assuming the nominal answer persists.
Worked decision context
Z0 = 50 Ω and driver resistance 15 Ω give a first-pass source resistor of 35 Ω. That example verifies the equation and illustrates the scale of the result, but it does not select a part by itself. Compare the result with available values, ratings, tolerance bands, and the receiving circuit or physical environment before implementation.
Use the primary output to identify the binding constraint. If it leaves little margin, document which input dominates and use selected-part data, a higher-fidelity model, simulation, or measurement. This is especially important when a small numerical difference changes a thermal, timing, noise, or reliability decision.
Limits and validation handoff
This assumes a purely resistive, level-independent driver output on an unloaded line. Real driver resistance varies with output level, and stub loads change the ideal value. The calculation does not silently include omitted parasitics, installation conditions, manufacturing variation, or product policy. Those conditions can be decisive even when the arithmetic is exact for the selected model.
Record inputs, units, model assumptions, and the intended decision with the result. Verify the leading risk against the selected component data sheet and a representative measurement when the circuit has consequential energy, high voltage, safety, compliance, or reliability requirements.
Common mistakes
- Treating the source-termination result as a guaranteed operating limit rather than a first-pass standard approximation estimate.
- Mixing a data-sheet value measured under one condition with this source-termination calculation performed at another.
- Selecting a component before checking the boundary this calculation names: this assumes a purely resistive, level-independent driver output on an unloaded line. Real driver resistance varies with output level, and stub loads change the ideal value.
Model limit and handoff
Keep the entered source-termination conditions with the calculation, then validate the binding limit using the selected component, physical implementation, and representative operating corner.
FAQs
Is this source-termination result sufficient to approve a design?
No. It applies standard approximation reasoning to the entered source-termination values and names the checks that still need selected-part data, a higher-fidelity model, or measurement. This assumes a purely resistive, level-independent driver output on an unloaded line. Real driver resistance varies with output level, and stub loads change the ideal value.
What does this source-termination calculator assume that could make the result wrong?
First-pass series resistor is line impedance minus driver output resistance, so the combined source impedance matches the line. If the entered values do not match the real operating condition, the result no longer describes the actual circuit.
Where should this source-termination result go next?
Compare this source-termination result with When traces become transmission lines, then use the stated next decision below the calculator to move from this first-pass number toward an implementation.