Data Acquisition

mines the amount of noise in the circuit. The preferred shield grounding configurationwhen there is no other alternative is shown in Figure 3.40, in which a portion of theground loop current is bypassed through the lower impedance shield.Figure 3.40Preferred shield grounding when measuring a grounded source with a grounded measurement systemBreaking the ground loop on the signal lines using transformers or optical couplers canprovide additional noise reduction.The rules of shield grounding are as follows:• Where possible, cable shields should be earthed at one end only.• Where the source is ungrounded and the signal amplifier is grounded, theinput shield should always be connected to the amplifier common terminal,even if this point is not at earth ground.• Where the source is grounded and the signal amplifier is ungrounded, theinput shield should be connected to the source common terminal, even if thispoint is not at earth ground.Grounding the shield has additional benefits such as providing a path for RF currentsand preventing the build-up of static charge by providing a discharge path to ground.Cables with copper conductors and plastic insulation are still the most common andreliable solution. This is not surprising as they combine the important elements of goodelectrical characteristics, low cost, mechanical flexibility, ease of installation and ease oftermination. Aluminum conductors are seldom used for data communication cablesbecause of the higher resistance and other physical limitations.The cable resistance depends on the cross-sectional area of the conductor (usuallyexpressed in mm 2 ) and the length of the cable. The thicker the conductor, the lower theresistance, the lower the signal volt drop, and the higher the current it can carry withoutexcessive heating.The signal voltage drop, V drop =I (R+(2 π f L – 1/2 πf C)), depends on the:• Frequency of signal• Line current, which is dependent on the receiver input impedance, and• Conductor resistance, which is dependent on wire size and length.For DC voltages and low-frequency signals the resistance of the conductor is the onlymajor concern. The voltage drop along the cable affects the magnitude of the signal volt-