Transformerless Ethernet using Capacitive Coupling in Embedded and Backplane Systems
Ethernet interfaces traditionally use magnetics (transformers) to provide galvanic isolation and common-mode noise rejection. However, in tightly integrated embedded systems or backplane designs, transformerless Ethernet becomes desirable due to:
• Board space limitations
• Cost and height constraints
• Lack of isolation requirement (e.g., shared ground plane)
This note explains the principles of transformerless Ethernet using AC coupling, with a focus on the differences between voltage-mode and current-mode PHY drivers, and details practical implementation strategies.
Recommendations
1. Choose Voltage-Mode PHYs with Internal Termination and AC-Coupling Support
2. Use 1000BASE-KX or 1000BASE-CX PHYs for backplane and internal Ethernet
3. Avoid transformerless configurations with 1000BASE-T or current-mode PHYs unless the design is well characterized.
4. Validate mixed configurations using simulation tools or prototype testing.
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