I want to make sure I understand what I am reading in various spec sheets when they quote their noise measurements and I hope that you-all can confirm that I am on the right track. Take the OP27 Op-Amp as an example, which is described as a low-noise precision operational amplifier. According to the spec sheet, http://www.analog.com/static/imported-file...Sheets/OP27.pdf This op-amp has a noise level of 3 nV/√Hz (This is the relevant noise value to consider, right? How does the Noise Figure factor in?) In the best case scenario where there is zero input and all the output that I measure is purely inherent noise in the device, If I wanted to find out how much noise power I would measure then I would need to know two things: across what impedance the measurement is taken, and over what bandwidth. Let’s say that I measure across the 1 MOhm (ideal) resistor in my oscilliscope, and measure over the full 8 MHz bandwidth of the op-amp. In which case the noise power would be: P_noise = (Bandwidth) * (V_noise)^2 / R P_noise = (8 MHz) * (3 nV/√Hz)^2 / (1 MOhm) P_noise = 7.2 E-17 Watts = 72 aW = -131 dBm Things that would increase this noise are influences such as the Johnson–Nyquist noise across the real (non-ideal) resistors that I use to take the measurement. This Johnson noise would be: P = k * T * Δf = about 32 fW = -105 dBm at room temperature for the same 8 MHz bandwidth, which is inescapable and far exceeds the noise from the op-amp. If I wanted to lower this noise value further then I could, for example, limit the bandwidth over which I measure. Instead of 8 MHz I might measure over only 1 MHz frequency range and thereby get only 1/8 of the above-calculated noise values. Is all this correct? Now let’s say, hypothetically, that I have a very tiny signal that I am amplifying with this op-amp. For the sake of example, say I want to amplify a signal that input power of -131 dB. There would be no hope that this would work since my signal is at the same level as the background noise (even ignoring the Johnson noise). Or does the frequency and bandwidth of the signal matter? What if I increased the input power to -105 dBm. Now the signal will go through the amplifier alright, but it is buried in the Johnson noise. Finally, increase the signal to something “big”, like -50 dBm. Now the signal is well above the op-amp and Johnson noise and could be read just fine by some ideal measuring apparatus.