In a floating source, the voltage signal is not connected to any absolute reference or any common ground, such as earth or building ground as shown in the following figure.
Floating signal sources are also called nonreferenced signal sources. Some common examples of floating signal sources are batteries, thermocouples, transformers, and isolation amplifiers. Notice in the figure that neither terminal of the source is connected to the electrical outlet ground, so each terminal is independent of the system ground.
You can measure floating signal sources with both differential and single-ended measurement systems. In the case of the differential measurement system, however, make sure the common-mode voltage level of the signal with respect to the measurement system ground remains in the common-mode input range of the measurement device. A variety of phenomena—for example, the instrumentation amplifier input bias currents—can move the voltage level of the floating source out of the valid range of the input stage of a DAQ device.