Noise Figure

Every resistor produces an irreducible thermal (Johnson-Nyquist) noise voltage of √(4kTRB) — about 4 nV/√Hz from a 1 kΩ resistor at room temperature. On top of this floor, real resistor materials add a 1/f “current noise” component that scales with applied DC voltage and depends on construction. This excess noise is what datasheets call the noise index, given in dB·μV/V or equivalently in μV/V at 1 V applied.

Wirewound and bulk metal foil resistors are the quietest: their excess noise is essentially zero (NI ≤ −40 dB). Metal film parts fall around −20 to −30 dB. Thick film typically runs −15 to −25 dB, and old-school carbon composition can be as bad as +10 dB — visibly noisy in audio gear and low-level instrumentation. Surface defects, contact resistance and chip resistor terminations all add to the figure.

Noise matters in three domains: precision DC measurement (where 1/f noise determines the long-term floor of voltage references and op-amp circuits), audio gain stages, and low-level sensor front-ends. For pure power-dissipation roles noise is irrelevant. Engineers minimise noise by choosing wirewound or foil parts in feedback and gain-setting positions and never relying on a thick-film part as the sole element in a precision divider.