PCB Differential Pair Impedance Calculator

Differential Impedance Formula:

\[ Z_{diff} = \frac{120}{\sqrt{\epsilon_r}} \times \ln\left(\frac{1.9(2h + t)}{0.8w + t}\right) \times \left(1 - 0.347 e^{-\frac{2.9s}{h}}\right) \]

Relative Permittivity (εr):

Dielectric Height (h):

Trace Thickness (t):

Trace Width (w):

Trace Spacing (s):

Differential Impedance (Z_diff):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is PCB Differential Pair Impedance?

Definition: Differential impedance is the impedance between two conductors when driven with opposite polarity signals, crucial for high-speed digital designs.

Purpose: Proper impedance matching ensures signal integrity, minimizes reflections, and reduces electromagnetic interference in PCB designs.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ Z_{diff} = \frac{120}{\sqrt{\epsilon_r}} \times \ln\left(\frac{1.9(2h + t)}{0.8w + t}\right) \times \left(1 - 0.347 e^{-\frac{2.9s}{h}}\right) \]

Where:

\( Z_{diff} \) — Differential impedance (ohms)
\( \epsilon_r \) — Relative permittivity of dielectric material
\( h \) — Height of dielectric substrate (mm)
\( t \) — Thickness of copper trace (mm)
\( w \) — Width of trace (mm)
\( s \) — Spacing between traces (mm)

3. Importance of Differential Impedance

Details: Controlled differential impedance is essential for high-speed interfaces like USB, HDMI, PCIe, and DDR memory buses to maintain signal quality.

4. Using the Calculator

Tips: Enter the PCB parameters in millimeters. Typical values are provided as defaults. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What's a typical differential impedance value?
A: Common values are 90Ω (USB), 100Ω (Ethernet), and 85-100Ω (HDMI), depending on the standard.

Q2: How does trace spacing affect impedance?
A: Closer spacing reduces impedance due to increased coupling between traces.

Q3: What εr value should I use for FR4?
A: FR4 typically has εr ≈ 4.3, but this can vary with frequency and specific material grade.

Q4: Why is trace thickness important?
A: Thicker traces have lower impedance due to increased cross-sectional area.

Q5: How accurate is this calculator?
A: This provides a good estimate, but for production designs use field solvers or consult your PCB manufacturer.