For instance, comparing the MIC920 and the BGA2818, it's expected that since the latter is intended for application as an RF LNA and the former is general-purpose, the BGA2818 would out-perform the MIC920 in RF applications. But how could this be proven?
The MIC920 specifies only an input noise voltage; let's assume they intended to write nV instead of V:
And the BGA2818 specifies only an input noise figure:
The "naive" comparison would be (following the Maxim App Note 2895):
BW for my application = 5 kHz
Gain of MIC920 ~ 85dB
Output power noise density: $$V_{IN} = \left( 11 \frac {\text {nV}} {\sqrt{\text {Hz}}} \right) \sqrt{5 \text{kHz}} = 778 \text {nV} $$ $$ P_{IN} = 10 \log \left( \frac {778 \text {nV}} {50 \Omega \cdot 10^{-3}} \right) = -48.1 \text{dBm} $$
$$P_{OUT} = -48.1 \text{dBm} + 85 \text{dB} = 36.9\text{dBm}$$
$$P_{NOUTD} = \frac {36.9\text{dBm}} {5\text{kHz}} = 7.38 \times 10^{-3} \frac {\text{dBm}} {\text{Hz}}$$
$$NF = 7.38 \times 10^{-3} \frac {\text{dBm}} {\text{Hz}} + 174 \frac {\text{dBm}} {\text{Hz}} - 85 \text{dB} = 89 \text{dB}$$
Is this calculation valid?