# How can I extract the rms noise from the measurement's over all noise?

To clarify my question, here is an example: Let's say I have an opamp and I want to measure and quantify its output noise when its input is grounded. So after shorting the opamp's input to its ground, I couple its output to an oscilloscope and read the rms voltage. Call it $$\V_\mathrm{RMS_{all}}\$$.

Now some part of the the noise I read at scope screen is related to inherent noise from the scope itself. Call that $$\V_\mathrm{RMS_{scope}}\$$.

1. What is the practical way to extract only the opmap output noise ($$\V_\mathrm{RMS_{signal}}\$$) in this case? Should I short the scope inputs(and call the measurement Vrms_scope) and subtract it from the $$\V_\mathrm{RMS_{signal}}\$$? Does the following formula make sense: $$V_\mathrm{RMS_{signal}}^2 = V_\mathrm{RMS_{all}}^2 - V_\mathrm{RMS_{scope}}^2$$
2. Does such a method neglect common mode related noise?
• Add a 20dB gain stage before the scope. Measure its own noise with shorted input before adding the DUT. If that's not clean enough, repeat (40 or 60dB). Feb 28 '20 at 14:18