


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
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



