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?
Best Answer
Yes but make sure you are still grounded to where you want to measure the noise from your amplifier - there may be some common-mode artefacts that bump up the noise even when the scope inputs are shorted to each other and, if the scope inputs were not grounded (but floating) you might not capture them (if any).
If you keep the ground, grounded it should be more accurate. The formula is correct by the way and take several measurements to ensure agreement.