PCB Differential Impedance Calculator

Differential Impedance Formula:

\[ Z_{diff} = 2 \times Z_0 \times (1 - 0.48 e^{-0.96 d / h}) \]

Differential Impedance (Z_diff): ohms

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1. What is PCB Differential Impedance?

Definition: Differential impedance is the impedance between two conductors when driven with equal but opposite signals, critical for high-speed digital and RF circuit design.

Purpose: Proper differential impedance matching ensures signal integrity, reduces EMI, and minimizes reflections in differential pair transmission lines.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ Z_{diff} = 2 \times Z_0 \times (1 - 0.48 e^{-0.96 d / h}) \]

Where:

\( Z_{diff} \) — Differential impedance (ohms)
\( Z_0 \) — Characteristic impedance of single trace (ohms)
\( d \) — Separation between traces (meters)
\( h \) — Height above reference plane (meters)

Explanation: The formula accounts for the coupling effect between traces, which reduces the differential impedance compared to simply doubling the single-ended impedance.

3. Importance of Differential Impedance

Details: Proper differential impedance matching is crucial for high-speed interfaces like USB, HDMI, PCIe, and Ethernet to maintain signal integrity and minimize crosstalk.

4. Using the Calculator

Tips: Enter the characteristic impedance of a single trace, the separation between traces, and the height above the reference plane. All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What's a typical differential impedance value?
A: Common values are 90Ω for USB, 100Ω for Ethernet, and 85-100Ω for HDMI, though requirements vary by application.

Q2: How does trace separation affect impedance?
A: Closer traces increase coupling, reducing differential impedance. The effect diminishes as separation exceeds about 3× the height above plane.

Q3: What dielectric height should I use?
A: This is the distance from the trace to the nearest reference plane (usually ground) in your PCB stackup.

Q4: How accurate is this formula?
A: This is an approximation. For precise results, use field solver tools that account for trace width, thickness, and dielectric constant.

Q5: Does this work for microstrip and stripline?
A: This formula is primarily for microstrip. Stripline configurations require different calculations due to different field distributions.