Johnson noise calculator

Assumptions to check

• The entered Johnson-noise values represent the stated operating condition.
• Values are evaluated in the declared lumped or first-pass model.
• A nominal calculation is not a component qualification or safety approval.

What this Johnson-noise calculation establishes

Calculate ideal resistor thermal-noise voltage density, integrated RMS noise, and available kTB noise power. The useful result is the stated electrical quantity and the decision it supports, not an unstated claim about a finished product. This engine keeps the governing relationship visible so an input, unit, condition, or model boundary can be reviewed before a value becomes a component or layout choice.

Integrated resistor noise is the square root of four times Boltzmann’s constant, temperature, resistance, and equivalent noise bandwidth. Treat the number as a first-pass result for the declared operating point. When a source, load, temperature, frequency, waveform, component tolerance, or measurement condition changes, repeat the calculation at the relevant corner rather than assuming the nominal answer persists.

Worked decision context

At 300 K, 1 kΩ over 20 kHz has 0.5756 µV RMS Johnson voltage noise; available kTB power is independent of resistance. That example verifies the equation and illustrates the scale of the result, but it does not select a part by itself. Compare the result with available values, ratings, tolerance bands, and the receiving circuit or physical environment before implementation.

Use the primary output to identify the binding constraint. If it leaves little margin, document which input dominates and use selected-part data, a higher-fidelity model, simulation, or measurement. This is especially important when a small numerical difference changes a thermal, timing, noise, or reliability decision.

Limits and validation handoff

It omits amplifier noise, filter shape beyond the entered equivalent bandwidth, aliasing, impedance interaction, and measurement uncertainty. The calculation does not silently include omitted parasitics, installation conditions, manufacturing variation, or product policy. Those conditions can be decisive even when the arithmetic is exact for the selected model.

Record inputs, units, model assumptions, and the intended decision with the result. Verify the leading risk against the selected component data sheet and a representative measurement when the circuit has consequential energy, high voltage, safety, compliance, or reliability requirements.

Common mistakes

• Treating a nominal result as a guaranteed operating limit.
• Mixing a data-sheet value from one condition with a calculation at another.
• Selecting a component before checking rating, tolerance, and the physical implementation.

Model limit and handoff

Keep the entered Johnson-noise conditions with the calculation, then validate the binding limit using the selected component, physical implementation, and representative operating corner.

FAQs