There are two types of differential probes, high voltage type and transistor based active. The first example covers some high voltage type differential probes, in an application where there is no ground reference, for example when dealing with a power supply. In this example is a pair of power MOSFETs, like the upper gate and the lower gate displayed in the lower right corner of this slide. There is a total of 350V across the two MOSFETs. It might be necessary to probe across one of these FETs, which would be an un-referenced measurement. There are several ways to do this, some of them are not safe. Unlike a hand held DMM, a general purpose oscilloscope is only capable of ground referenced measurement. This is because the reference lead of the measurement, the probe ground lead, is directly connected to the case of the scope. The case is also connected to earth ground through the protective ground wire in the scope’s power cable. This second ground path allows a waveform to be seen on the scope when only the center tip of the probe is touched to the circuit being measured. (Note that signals with high frequency components will be severely degraded when the probe ground lead is not connected.) For this reason, measurements made with general purpose oscilloscopes are often referred to as “single ended”. While this configuration is adequate for many routine measurements, there are several applications where this restriction degrades the measurement quality, or prevents the measurement from being made altogether.