A differential voltage is “floating”, meaning that it has no reference to ground. The measurement is taken as the voltage difference between the two wires. The main benefit of a differential measurement is noise rejection, because the noise is added to both wires and can then be filtered out by the common mode rejection of the data acquisition system. Differential measurements should be used if the sensor is in a noisy environment or for sensors with output voltages susceptible to noise interference. For example, we recommend that the thermopile output from Apogee SI-100 series infrared radiometers should always be measured differentially because the small voltages are susceptible to noise.