Originally Posted By: The_Eric
My electronics knowledge is a bit lacking- would you guys explain the relevance or importance of impedance in a lab scope?
Loading of the circuit under test. A mechanical analogy might be, say, a tachometer that takes oh say 0.25hp to operate. On most any internal combustion engine, 0.25hp is negligible. But on a 0.5hp electric motor that would be a massive load. It might be enough that the motor can no longer spin at the specified rpm.
Many lab circuits are not zero output impedance: any sort of loading will cause the voltage to drop, and often very appreciably. Sometimes just hooking up a scope is enough to stop the circuit from working altogether (in some cases it might start working--until you remove the scope!). Or the scope can lie to you: it causes loading, but the circuit doesn't care about the loading--but the scope "sees" one thing and the circuit "sees" something else. Kind of like if you tape a thermocouple to the outside of an oil pan. The thermocouple sees oil temperature, but it's not the same as what's inside the pan (much less what the oil temp is on the cylinder wall).