Ethernet PCB Design Guide

Design reliable 1G, 10G, and 25G Ethernet interfaces. Learn PHY layout, magnetics placement, and differential pair routing for high-speed networking.

Calculate Impedance Differential Calculator

Ethernet Design Checklist

100Ω differential impedance (±10%)
Magnetics close to RJ45 (<25mm)
Bob Smith termination for EMI
ESD protection on line side
Length match pairs within 50 mils

Ethernet Standards

Standard	Speed	Pairs	Impedance	Frequency	Cable
100BASE-TX	100 Mbps	2	100Ω diff	31.25 MHz	Cat5
1000BASE-T	1 Gbps	4	100Ω diff	62.5 MHz	Cat5e
2.5GBASE-T	2.5 Gbps	4	100Ω diff	100 MHz	Cat5e
5GBASE-T	5 Gbps	4	100Ω diff	200 MHz	Cat6
10GBASE-T	10 Gbps	4	100Ω diff	400 MHz	Cat6a
25GBASE-T	25 Gbps	4	100Ω diff	1 GHz	Cat8

Ethernet Layout Guidelines

PHY Placement

• Place PHY between MAC and connector
• Short MDI traces to magnetics
• Separate analog and digital supplies
• Follow PHY vendor layout guidelines

Magnetics & Connector

• Use integrated magjack when possible
• 1:1 or 1:2.5 center-tapped transformers
• Common mode chokes for EMI
• Shield RJ45 to chassis ground

Differential Pairs

• 100Ω differential, 50Ω single-ended
• Keep pairs tightly coupled
• Match lengths within each pair
• Avoid crossing other high-speed signals

Speed-Specific Design Tips

1G Ethernet (1000BASE-T)

Standard FR-4 is sufficient
All 4 pairs used for TX/RX simultaneously
Auto-MDIX handles crossover
Echo cancellation in PHY compensates crosstalk

10G+ Ethernet

Consider low-loss PCB material
PAM-16 encoding for 10GBASE-T
Minimize return loss at connector
Higher power dissipation - thermal design

FAQ

Where should I place Ethernet magnetics?

Place magnetics as close to the RJ45 connector as possible - typically within 25mm. This minimizes trace length on the line side (exposed to ESD) and maximizes EMI suppression. Many designs use integrated magjack connectors combining RJ45 and magnetics in one module.

What is the impedance requirement for Ethernet?

Ethernet uses 100Ω ±10% differential impedance for all speeds from 100Mbps to 25Gbps. The MDI interface (PHY to magnetics) and cable both require 100Ω. Use tightly coupled differential pairs and maintain consistent impedance through connectors and vias.

How do I route 10GBASE-T signals?

10GBASE-T uses all 4 pairs with PAM-16 encoding. Keep pairs tightly coupled with 100Ω impedance. Match pair lengths within 50 mils. Route directly from PHY to magnetics with minimal vias. Use solid ground reference and consider low-loss materials for longer traces.