Signal Integrity5 min read
Twisted Pair Wiring: Noise Rejection Explained
How twisted pairs cancel electromagnetic interference for clean signal transmission.
Quick Definition
Twisted pair is two insulated wires twisted together at a regular rate. The twist causes electromagnetic interference to affect both wires equally, allowing differential receivers to cancel out the noise. Used in Ethernet, CAN bus, USB, and audio applications.
How Twisted Pair Works
The Physics
- 1. External EMI induces equal voltage in both wires
- 2. The twist ensures each wire is equally exposed to the interference
- 3. The receiver measures the difference between the two wires
- 4. Equal noise on both wires = difference of zero = noise cancelled
Key Factors
- • Twist rate: More twists = better HF rejection
- • Balance: Wires must be identical
- • Termination: Keep untwisted length minimal
- • Shielding: Optional, for harsh environments
Twisted Pair Applications
| Application | Twist Rate | Impedance | Shielding |
|---|---|---|---|
| Ethernet (Cat5e/Cat6) | 2-3 per inch | 100Ω | UTP or STP |
| CAN Bus | ~1 per inch | 120Ω | Usually shielded |
| RS-485 | ~1 per inch | 120Ω | Often shielded |
| USB | Varies | 90Ω | Shielded |
| Audio (Balanced) | ~0.5 per inch | N/A | Shielded |
| Telephone (POTS) | ~0.5 per inch | 100Ω | UTP |
UTP vs STP
UTP (Unshielded)
- ✓ Lower cost
- ✓ Easier to terminate
- ✓ More flexible
- ✓ Sufficient for most environments
- ✗ Less EMI protection
Best for: Office, residential, light industrial
STP (Shielded)
- ✓ Superior EMI rejection
- ✓ Required near VFDs, motors
- ✓ Better for long runs
- ✗ Higher cost
- ✗ Requires proper grounding
Best for: Industrial, manufacturing, high-EMI
⚠️ Common Mistakes
Excessive untwisting at termination
Keep untwisted length under 0.5" (13mm) for Ethernet. Untwisted sections don't benefit from noise cancellation.
Running next to power cables
Maintain 12"+ separation from high-voltage lines. Cross at 90° angles when unavoidable. Use STP if in doubt.
Frequently Asked Questions
Related Glossary Terms
Design Signal-Integrity Harnesses
WireIt helps you specify twisted pairs and shielding for reliable data transmission.