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}}\$.
- 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: $$ V_\mathrm{RMS_{signal}}^2 = V_\mathrm{RMS_{all}}^2 - V_\mathrm{RMS_{scope}}^2 $$
- Does such a method neglect common mode related noise?