Differential Signaling & EMI/EMC
Most robust/fast links (CAN, RS-485, USB, Ethernet, LVDS, HDMI, MIPI) are differential. This page explains why, and how it ties into EMI/EMC — what causes electromagnetic noise and how differential signaling fights it.
Single-ended vs differential​
- Single-ended (UART, I²C, SPI, plain GPIO): one wire carries the signal, measured against a shared ground. Simple, but any noise or ground shift rides straight into the reading.
- Differential: two wires carry equal and opposite signals; the receiver reads the difference between them. Noise that hits both wires equally (common-mode) cancels out.
Pros & cons​
| Pros | Cons |
|---|---|
| Common-mode noise rejection → high immunity | Two wires/pins instead of one |
| Lower emissions (the pair's fields cancel) | Needs matched routing (length + impedance) |
| Tolerates ground-potential differences | Needs transceivers |
| Faster / longer reach | More PCB care (pairs, planes) |
How it helps EMC (both directions)​
- Emissions: equal-and-opposite currents create fields that cancel, so a tight pair radiates far less than a single-ended trace + its return loop.
- Immunity: external interference couples onto both lines about equally; the receiver subtracts it away (common-mode rejection).
You only get these benefits if the pair is tightly coupled and balanced — route as a pair, length-matched, controlled impedance (90/100 Ω), over a continuous reference plane. A common-mode choke removes residual common-mode noise on cables.
Sources of EMI (what generates the noise)​
- Switching regulators — fast switching nodes and their current loops.
- Fast digital edges & clocks — sharp edges carry lots of high-frequency energy.
- Large current loops — act as loop antennas (a return-path problem).
- Inductive loads — motors, relays, solenoids (switching spikes).
- ESD and external surges.
- Cables — long traces/cables behave as antennas; ground bounce drives them.
Effects of EMI (why you care)​
- Corrupted bits / CRC errors, unexplained resets, noisy ADC readings, and failed EMC compliance (a product can't ship until it passes).
Mitigation summary​
- Prefer differential for anything off-board or noisy (CAN, RS-485, USB, Ethernet).
- Shrink current loops; keep solid reference planes; never cross a plane split.
- Slow unneeded edges; keep switching-regulator loops tight.
- Filter and protect cables/connectors: common-mode chokes, ferrites, TVS/ESD.
This is the firmware ↔ PCB bridge
Choosing a differential bus is a firmware/system decision; realising its EMC benefit is a PCB layout job. See EMI / EMC and High-Speed Digital Signals.