Question 23
There are times when you need to use an oscilloscope to measure a differential voltage that also has a significant common-mode voltage: an application where you cannot connect the oscilloscope's ground lead to either point of contact. One application is measuring the voltage pulses on an RS-485 digital communications network, where neither conductor in the two-wire cable is at ground potential, and where connecting either wire to ground (via the oscilloscope's ground clip) may cause problems: