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Differential pair

Differential pair: the example drawn by the calculatorw = 0.3 mms = 0.2 mmh = 0.2 mmground planeZdiff ≈ 88.1 Ω
Zdiff = 88.1 Ω: 0.3 mm traces, 0.2 mm apart, 0.2 mm over the plane

Two traces side by side pull each other's impedance down: the closer, the lower.

Opens in the app on 1 December, with Circuit Notebook 1.0 See it worked out

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The example, worked out

Given

Where the pair runs
Outer layer (microstrip)
I want
Zdiff of a width and a gap
Trace width
0.3 mm
Gap between the traces
0.2 mm
Dielectric height
0.2 mm
Permittivity εr
4.4
Copper thickness
35 µm

Working

  1. one trace over the plane (Hammerstad, as in the microstrip calculator)
  2. the neighbour pulls it down

Results

Differential impedance Zdiff88.1 Ω
One trace alone Z053.97 Ω
Odd mode Zodd44.05 Ω
Pulled down by the neighbour18 %
Delay6.06 ps/mm

The formulas

  1. Zdiff ≈ 2 Z0 (1 − 0.48 e^(−0.96 s/h))microstrip: the neighbour pulls Z0 down
  2. Zdiff ≈ 2 Z0 (1 − 0.347 e^(−2.9 s/b))stripline, between two planes
  3. Zodd = Zdiff / 2one trace while the other swings the other way

The usual traps

  • Zdiff is not 2 × Z0 unless the traces are far apart.
  • Changing the gap along the route changes the impedance: keep s constant, even around corners.
  • The closed forms drift by a few percent: the fab's stack-up and field solver have the last word.

Questions

How do I get 90 Ω or 100 Ω on a differential pair?

Zdiff ≈ 2 Z0 (1 − 0.48 e^(−0.96 s/h)): microstrip: the neighbour pulls Z0 down. Zdiff ≈ 2 Z0 (1 − 0.347 e^(−2.9 s/b)): stripline, between two planes. Zodd = Zdiff / 2: one trace while the other swings the other way.

What does it give for typical values?

Zdiff = 88.1 Ω: 0.3 mm traces, 0.2 mm apart, 0.2 mm over the plane. Differential impedance Zdiff: 88.1 Ω.

What are the usual mistakes?

Zdiff is not 2 × Z0 unless the traces are far apart. Changing the gap along the route changes the impedance: keep s constant, even around corners. The closed forms drift by a few percent: the fab's stack-up and field solver have the last word.

In the app

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Results rounded to standard values (E12, E24, E96) and power ratings.

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Opens in the app on 1 December, with Circuit Notebook 1.0

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