Noise Calculator
Calculate thermal, shot, and total noise in analog circuits. Determine SNR and noise floor.
Key Formulas
Thermal: Vn = √(4kBTR×BW)
Shot: In = √(2qIDCBW)
Total: Vn_total = √(ΣVn²)
Frequently Asked Questions
What types of noise does this calculator compute?
The Noise Calculator computes thermal (Johnson-Nyquist) noise from resistance, shot noise from DC current, and total integrated input-referred noise. It also estimates signal-to-noise ratio (SNR) and system noise floor in volts RMS, enabling quick assessment of analog circuit sensitivity and dynamic range.
When should I use this tool in my circuit design workflow?
Use it during front-end analog design—especially for sensor interfaces, precision amplifiers, ADC drivers, and low-level signal conditioning. It helps validate noise budgets early, compare op-amp candidates, and determine if bandwidth limiting or gain staging is needed to meet SNR targets.
What does “Op-Amp Noise (nV/√Hz)” represent, and how do I find its value?
This is the op-amp’s input voltage noise spectral density—typically specified in the datasheet under “Input Voltage Noise” or “en”. For bipolar-input op-amps, expect 1–10 nV/√Hz; for JFET or CMOS inputs, 5–40 nV/√Hz. Use the 1 kHz or 10 kHz value unless your application is DC-critical (then check 0.1–10 Hz flicker noise separately).
Why is temperature entered in °C but used in Kelvin for calculations?
Thermal noise depends on absolute temperature (T in Kelvin), so the tool automatically converts your °C input using T(K) = T(°C) + 273.15. Room temperature (25°C → 298.15 K) is the default because it reflects standard lab and operational conditions for most analog designs.
What are typical realistic ranges for the input parameters?
Resistance: 1 Ω – 10 MΩ (1–10,000 kΩ); Bandwidth: 0.1 Hz – 100 MHz (0.0001–100,000 kHz); DC Current: 0.001–100 mA; Op-Amp Noise: 0.5–50 nV/√Hz; Gain: 1–1000. Values outside these ranges may indicate modeling limitations (e.g., neglecting 1/f noise or parasitic capacitance).
My calculated total noise seems too high—what should I check first?
Verify units: resistance in kΩ (not Ω), bandwidth in kHz (not Hz), and current in mA (not A). Also confirm whether your op-amp’s specified noise is *voltage* noise only—if current noise (in) is significant (e.g., with high source impedance >100 kΩ), it must be added in quadrature and isn’t modeled here.
Does this calculator include 1/f (flicker) noise?
No—it assumes white (frequency-independent) noise over the specified bandwidth. For low-frequency applications (<1 kHz), 1/f noise can dominate. To estimate its impact, refer to your op-amp’s noise corner frequency and integrate en²(f) manually, or use a more advanced noise simulator like LTspice with vendor-provided noise models.
How is SNR calculated, and what reference signal level does it assume?
SNR is computed as 20·log₁₀(Vsignal,RMS/Vnoise,RMS), where Vsignal,RMS is derived from the full-scale output divided by your gain (AV). It assumes a sine-wave input at maximum undistorted output—so SNR reflects best-case performance before clipping or distortion limits apply.
Can I use this calculator for current-output sensors (e.g., photodiodes)?
Yes—but interpret results carefully. For transimpedance amplifiers, enter the feedback resistance (Rf) and bandwidth (set by RfCf), and include shot noise from photocurrent. Note that op-amp current noise becomes critical here and requires separate calculation not covered in this tool.
What’s the difference between “noise floor” and “total integrated noise” in the output?
“Total integrated noise” is the RMS voltage noise across your specified bandwidth—calculated by integrating all noise sources. The “noise floor” is the same value expressed in dBm or dBV (often dBV RMS), enabling direct comparison with signal levels, ADC resolution, or spectrum analyzer displays.