Oscilloscope users often need to make "floating" measurements where neither point of the measurement is at ground (earth) potential. "Signal common" may be elevated to hundreds of volts from earth. In addition, many of these measurements require the rejection of high common-mode signals1 in order to evaluate low-level signals. Unwanted ground currents can also add bothersome hum and ground loops. Too often, users resort to the use of potentially dangerous measurement techniques to overcome these problems.
1 A "common-mode signal" is defined as a signal which is present at both points in a circuit. Typically referenced to ground, it's identical in amplitude, frequency, and phase. Making a floating measurement between two points requires rejecting the "common-mode signal" so the difference signal can be displayed.
51W-10640-1 2001-09-10 07:00:00