Data Acquisition

Examples of floating signal sources are transformers, isolation amplifiers, batteries, andbattery powered instruments.3.4.3 Single-ended measurementA ground-referenced measurement system, as shown in Figure 3.20, is one in which thevoltage measurement is taken with respect to ‘ground’. It is known as a single-endedmeasurement because only one signal line is required to determine the signal voltage,provided it is ground-referenced.Figure 3.20Single-ended measurement3.4.4 Differential measurementA differential measurement system, as shown in Figure 3.21, has neither of its inputs tiedto a fixed reference, such as earth or system ground.Figure 3.21Differential measurementDifferential measurement is beneficial because, noise induced equally into each of thesignal lines appears as a common mode voltage at the input and is largely rejected (seeCommon mode voltages and CMRR, below).3.4.5 Common mode voltages and CMRRCommon mode voltagesIdeally a differential measurement system measures only the potential difference betweenits positive and negative terminals. Where a signal source is measured using differentialinputs, and there is a voltage measured with respect to the measurement ground that ispresent on both input lines, then this voltage is referred to as a common mode voltage.This is shown in Figure 3.22