Noise ground loops

Because the output of these bridges is in the range of millivolts, the cables utilized to carry the signal are normally shielded with a braided or foil-lined sheath around individual wires. The shield, as a rule, is connected to the amplifier, but never touches the actual instrumented equipment (i.e., tablet press). If this rule is violated, a ground loop may generate electrical noise and present a dangerous electrical shock hazard. 

A frequent source of failure in TTS measurements is improper shielding of the detector. RF noise pickup from the environment or line frequency pickup via ground loops can severely impair the timing accuracy of the TCSPC electronics. It is obvious that the correct TTS cannot be measured under such conditions. 

Another major source of noise at low frequencies is mains pickup. This may amount to hundreds of millivolts superimposed on the test signal unless major efforts are made at shielding and ground loop suppression. Usually, unless an adequate notch filter is used in addition, the experimentalist must be satisfied with reduced precision at frequencies below about 100 Hz. 

Floating measurements are also used to avoid ground loops where equipment and test cell are grounded at different points leading to noise and interference in the measurements. If the cell is grounded but the equipment is floating then this problem can be avoided. 

Terminate signal grounds at one central location whenever possible to eliminate ground loops and noise. 

FIGURE 10.276 Ground-loop noise can be canceled by amplifying both the high side and the ground side of the source and subtracting the two signals. 

The move away from transformers introduced several unexpected problems. Installation practices that provide acceptable performance with transformer-isolated equipment may not work for active balanced I/O equipment. Active balanced circuits can be less forgiving of wiring faults and short circuits some output stages will self-destruct if shorted. Avoidance of ground loops is more important with an active balanced system than with transformers. Active balanced input circuits can provide excellent noise rejection under ideal conditions. In the real world, however, their noise rejection can deteriorate rapidly. Transformers remain the best choice for the toughest situations. 

This book can be used as a resource for working engineers and technicians, to quickly look up problems that commonly occur with industrial electronics, such as the measurement noise due to "EMI," or oscillations from "ground loops." Sufficient understanding can be obtained to solve such problems, and to avoid additional problems in the future. Most other books in this field are oriented toward electronics specialists, and they are more difficult for chemical, mechanical, or industrial engineers to use for this purpose. 

These are used where noise caused by the ground loop is undesirable and when total resistance of the loop exceeds the specification for the desired instrument application. 

Integration of analog and digital signal converters, especially with more than 12 bits of resolution—ground loops and noise... 

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