Data Acquisition

Figure 3.26Opto-coupler isolation of ground loopWhen a transformer is connected as a common mode choke, as shown in Figure 3.27,DC and differential analog signals are transmitted while common mode AC signals arerejected. The common mode noise voltage appears across the windings of the choke. Onebig advantage with this type of isolation circuit is that multiple signal circuits can bewound on a common core without coupling.Figure 3.27Common mode choke isolation of ground loop3.4.9 Measuring ungrounded signal sourcesUngrounded or floating signal sources can be measured using the single-ended, pseudodifferentialor differential measurement methods.Differential measurement of ungrounded signal sourcesWhen using the differential measurement system to measure the voltage signal from anungrounded source, care should be taken to ensure that the common mode voltage levelof the signal with respect to the measurement ground does not exceed the common modeinput voltage limits of the measurement device.In addition, where there is no return path to the measurement system earth for theinstrumentation amplifier input bias currents, then the flow of these currents through thesource impedance, as well as charging stray capacitances, can cause the voltage level ofthe source to float beyond the valid range of the input stage of the measurement system.This is especially true where the source impedance is high. The degree to which thesource voltage will float depends on the magnitude of the input bias currents and thesystem imbalance.A balanced measurement system meets the following criteria:• The input impedances to ground of each terminal of the instrumentationamplifier are equal.• The impedances of each signal cable to ground are equal.• The impedances to ground of each terminal of the source are equal.