Short Note: Shielded and Twisted-Pair Cables

Both reduce electromagnetic interference (EMI) picked up by signal cables.

Twisted pair

Two insulated wires twisted together. Interference induces equal voltages in both wires; a differential receiver cancels these (common-mode). Closer twists give better rejection, especially of magnetic fields.

Shielded cable

The conductors are surrounded by a metal foil or braid, grounded at one end (normally the signal source end), which blocks electric-field (capacitive) interference and prevents ground loops.

Point	Twisted pair	Shielded
Protects against	Magnetic and common-mode noise	Electric-field noise, RFI
Cost	Low	Higher
Use	Ethernet, RS-485	Low-level sensor signals, audio

Shielded twisted pair (STP) combines both and is used for thermocouples, ECG leads and RS-485.

Exam tip

Explain cancellation in twisted pair and single-point grounding of the shield.