Avian’s Blog

The two most common electrical measurements I do these days is measuring output impedance of a source and input impedance of a load. I don't have any special equipment for that. I'm not doing it so often that it would warrant buying or making a specialized tool. Since it's all either DC or around audio frequencies and I'm not interested in very precise values it's a really simple measurement. I usually just use an oscilloscope and whatever else is available around the shop to use as reference loads and sources.

Deriving unknown impedances or resistances from the voltage measurements and reference values is straightforward. It only takes a minute and I must have done it a hundred times at this point by inverting the voltage divider formula. I still don't know the resulting equations by heart though. So to avoid doing it the hundred and first time and the odd mistake, I've made myself a nice cheat sheet to stick above my desk. It contains the formulas for the three most common measurements I make.

PDF version

The notes about the best accuracy refer to the selection of the reference impedance such that the result is least affected by errors in voltage measurements (which, when measuring amplitudes with an oscilloscope, is the largest source of error in my case). The selection is quite obvious, but I guess I added it there for the sake of rigor.

The cheat sheet is made in LaTeX using the circuitikz package for schematics. I'm mostly putting it here for my own future reference, but maybe someone else will find it useful.