How this calculator works
Random thermal motion of charge carriers creates voltage fluctuations across a resistor. In the classical low-frequency limit the voltage-noise power density is 4kTR. Integrating a flat spectrum over equivalent noise bandwidth gives the RMS noise voltage. This is open-circuit voltage noise, not the voltage after a load divider. Excess current noise, flicker noise and quantum-frequency corrections are outside the model.
Formula
Vrms = sqrt(4 kB T R B); en = sqrt(4 kB T R) V/sqrt(Hz). dBV = 20 log10(Vrms/1 V); dBu = 20 log10(Vrms/sqrt(0.6) V).
Worked example
At 300 K, a 1000 ohm resistor over 10000 Hz generates 0.407035 microvolts RMS. Increasing resistance to 4000 ohms doubles the voltage noise to 0.814071 microvolts.
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Frequently asked questions
Can I enter Celsius?
Yes. The unit selector converts Celsius or Fahrenheit to kelvin before calculation. Absolute temperature must be positive.
Is bandwidth the filter cutoff?
Not necessarily. Use equivalent noise bandwidth, the width of an ideal rectangular filter passing the same integrated noise power.
Can better-quality resistors remove this noise?
All resistors at nonzero temperature exhibit thermal noise. Resistor construction can affect excess noise, which this model does not include.