These equations calculate
the noise figure of an opamp using parameters provided by vendors at the operating frequency and gain
configurations. Be sure to note units both for the input parameters and for the equations, or you will end up with
really bad results. I originally got these formulas from an engineering desk reference, but a source accessible by
everyone is "Noise Analysis for
HighSpeed Op Amps," by Texas Instruments (see page 9).
Approximation Formula
V_{NOISE }= Noise voltage density, i.e. nV/sqrt(Hz) I_{NOISE }= Noise current density, i.e.
pA/sqrt(Hz) R_{S} = Source resistance, i.e. 50 Ω
k = Boltzmann's Constant = 1.380 662 x 10^{23} J/K T = Ambient temperature in Kelvin
Complete NonInverting Opamp Noise Figure
Formula (for R_{S} = R_{T})
Complete Inverting Opamp Noise Figure Formula
(for R_{S} = R_{T})
E_{NI} = Input noise voltage I_{BN} = Noninverting input noise current
I_{BI} = Inverting noise current R_{S} = Source resistor R_{T}
= Termination resistor R_{F} = Feedback resistor R_{G} = Gain setting resistor
