The most desirable method is a true differential probe. As can be seen on this slide, one input channel of the oscilloscope is used. In this example the gate to the drain of the FET is being probed safely. This method has good common mode range, good common mode rejection ratio, and it is a true balanced probing technique. Also, the lead parasitic capacitance is cancelled out. All of the problems associated with floating the oscilloscope are overcome by using the proper differential signal conditioner. The oscilloscope remains safely grounded. Ground is not part of the measurement inputs.