A floating measurement reads the voltage between two points, neither of which is at ground potential. A most typical floating measurement done quite often is when doing voltage measurements with a battery powered multimeter. The floating insulated multimeter allows doing floating measurements of slowly changing signals easily.
The advent of switching power supplies and motor controllers brought a need to characterize waveshapes, timing, distortion, and other dynamics. Today’s power measurements call for an oscilloscope. Floating measurements of fast AC signals are a challenge for conventional instruments. The traditional line-powered benchtop oscilloscope models typically lack the possibility of making floating measurements. There are many compact battery-powered scopes around, but many of them have inadequate bandwidth and sample rate for accurate high-frequency waveform capture. Floating measurements have unique requirements over and above the usual considerations of bandwidth and resolution. Foremost among these issues is operator safety.
The Three Facets of “Floating” Measurement Solutions document at Tektronix web site examines the available alternatives for measuring AC signals in an ungrounded environment. It will show how the balance between three characteristics safety, packaging, and performance determine an instrument’s effectiveness for making floating measurements.