Delay-Matched Trace Length
Δl = vp × Δt
Quick answer
This calculator solves Δl = v_p × Δt for Length Difference. With Propagation Velocity 0.5, Skew Requirement 100 ps, the result is 5 × 10⁻⁸ mm. It solves in 3 directions, so you can enter the values you have and get the one you need.
Calculator
Formula
Description
Length matching converts a timing skew requirement into a maximum allowed trace length difference. For parallel buses (DDR, LVDS, parallel ADC interfaces), all data signals must arrive at the receiver within the setup/hold time window relative to the clock. The maximum allowable skew Δt translates to a physical length tolerance Δl based on the propagation velocity. Serpentine (meandering) routing is used to add length to shorter traces, equalizing the delay across all signals in a bus.
Variables
- Δl — Maximum trace length difference (m)
- v_p — Propagation velocity (m/s)
- Δt — Maximum allowable timing skew (s)
Practical Notes
For FR-4 microstrip (vp ≈ 165 mm/ns): 1 ps skew = 0.165 mm length. DDR4 requires data-to-strobe matching within about ±5 mm. USB 3.0 differential pair intra-pair skew must be < 5 mils (0.127 mm). HDMI skew requirement is ±2 mm within a pair and ±5 mm between pairs. When routing serpentine patterns, use smooth curves (not sharp corners) to minimize impedance discontinuities and crosstalk.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| dl | Length Difference | mm | mm, cm, m, km |
| vp | Propagation Velocity | — | — |
| dt | Skew Requirement | ps | ps, ns, µs, ms, s, min, h |
Step by step
- 1. What you know: Propagation Velocity 0.5, Skew Requirement 100 ps
- 2. The relation: Δl = v_p × Δt
- 3. Substitute: vp = 0.5, dt = 100 ps
- 4. Result: Length Difference = 5 × 10⁻⁸ mm
Worked examples
Minimum range
Given: Propagation Velocity 0.025, Skew Requirement 1 ps
Result: Length Difference = 2.5 × 10⁻¹¹ mm
Low range
Given: Propagation Velocity 0.25, Skew Requirement 10 ps
Result: Length Difference = 2.5 × 10⁻⁹ mm
Typical
Given: Propagation Velocity 0.5, Skew Requirement 100 ps
Result: Length Difference = 5 × 10⁻⁸ mm
High range
Given: Propagation Velocity 0.8, Skew Requirement 0.5 ns
Result: Length Difference = 4 × 10⁻⁷ mm
Maximum range
Given: Propagation Velocity 8, Skew Requirement 5 ns
Result: Length Difference = 4 × 10⁻⁵ mm
Solving for each variable
Solve for Length Difference
Propagation Velocity 0.025, Skew Requirement 1 ps → 2.5 × 10⁻¹¹ mm
Solve for Propagation Velocity
Length Difference 0.1 mm, Skew Requirement 1 ps → 1 × 10⁸
Solve for Skew Requirement
Length Difference 0.1 mm, Propagation Velocity 0.025 → 4 ms
How Length Difference changes with Propagation Velocity
| Propagation Velocity (input) | Length Difference (result) |
|---|---|
| 0.05 | 5 × 10⁻⁹ mm |
| 0.125 | 1.25 × 10⁻⁸ mm |
| 0.25 | 2.5 × 10⁻⁸ mm |
| 0.375 | 3.75 × 10⁻⁸ mm |
| 0.5 | 5 × 10⁻⁸ mm |
| 0.75 | 7.5 × 10⁻⁸ mm |
| 1 | 1 × 10⁻⁷ mm |
| 2 | 2 × 10⁻⁷ mm |
| 3.5 | 3.5 × 10⁻⁷ mm |
| 5 | 5 × 10⁻⁷ mm |
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<p><a href="https://vbatpower.com/formulas/delay-matched-length/">Delay-Matched Trace Length</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate Delay-Matched Trace Length?
Use Δl = v_p × Δt. With Propagation Velocity 0.5, Skew Requirement 100 ps, Length Difference works out to 5 × 10⁻⁸ mm.
What is the formula for Delay-Matched Trace Length?
The formula is Δl = v_p × Δt. Length matching converts a timing skew requirement into a maximum allowed trace length difference.
How do you find Propagation Velocity when you know the other values?
Rearrange for Propagation Velocity: with Length Difference 0.1 mm, Skew Requirement 1 ps, Propagation Velocity is 1 × 10⁸.
How do you find Skew Requirement when you know the other values?
Rearrange for Skew Requirement: with Length Difference 0.1 mm, Propagation Velocity 0.025, Skew Requirement is 4 ms.
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