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
High-Speed 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} = Non-inverting 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