I have heard people saying that you must place the signal injection resistor (10 to 100 R) between a high impedance point and a low impedance point. The usual place for voltage loop is between the output (low impedance) and the feedback resistor divider (high impedance).
Best place to inject signal for loop gain measurements?
Could someone explain the reasons behind this ? It seems logical to think that as long as the disturbance is injected into the loop, you should be able to measure the loop gain no matter where the injection takes place. I have had mixed success with this, sometimes it would work and sometimes not. So now I try to always inject directly after a buffer or the output.