Data Acquisition

of a circuit will drive a current of 2 A around the current loop if its resistance is 0.5 Ω.Where the current flow is significant, and the ground loop created by earthing of theshield has a large area, shield currents may inductively couple unequal voltages into thesignal cables and be a source of interference. Where possible, shields should be earthed atone end only.The placement of shield earths depends on the grounding of the signal source and thetype of measurement system used. Figure 3.38 shows the preferred shield groundingwhen measuring an ungrounded signal source, using a measurement system where thesignal lines are referenced to the amplifier common. It is assumed that amplifier common,although normally connected to ground may have a potential (∆Vg1) relative to groundpotential. ∆Vg2 represents the difference in ground potential. The circuit equivalent forthis system shows that in this configuration neither of the noise voltages (∆Vg1 or ∆Vg2)appears across the input terminals of the amplifier. Instead, if the shield was earthed atpoint B, then the noise voltage across the input terminals of the amplifier would be thevoltage across the impedance of C 2 as part of the voltage divider formed with C 1 .Figure 3.38Shield grounding when measuring an ungrounded source with a grounded measurement systemWhen an ungrounded (differential) measurement system is used to measure a groundedsource the preferred cable shielding is shown in Figure 3.39. The voltage ∆Vg1 representsthe potential of the source common above earth ground potential.Figure 3.39Shield grounding when measuring a grounded source with an ungrounded measurement systemThe equivalent circuit for this measurement system again shows that the noise voltageappearing across the input terminals of the amplifier, is zero. If the shield was groundedat the other end of the cable at point D, then the noise voltage across the input terminalsof the amplifier would be the voltage across the impedance of C 2 as part of the voltagedivider formed with C 1 .Where the signal circuit is required to be grounded at both ends, the difference inground potential and the susceptibility of the ground loop to inductive coupling deter-