GeneralInformation

Electrostatic capacitive couplingappears where wires run parallel toeach other, similar to magnetic cou- pling. That is where the similaritiesend.Electrostatic, or capacitive,coupling is a function of the distancethe wires run parallel to each other,the distancebetween the wires and wire diameters.

Application Hints

Common Impedance Coupling

How does electrical noise get in?

Common impedance couplingoccurs when two circuits share acommon conductor or impedance (even common power sources). Electromagnetic RadiationCoupling Magnetic Inductive Coupling Magnetic inductive coupling gener-ally appears where there are wires running parallel or in close vicinity toeach other. This happens when thewires from several different circuitsarebundled together in order tomake the system wiring appear neat. Helpful Wiring Guidelines Electromagnetic radiationcouplingoccurs when the sensor is very closeto a high energy source like TV orradio broadcasting towers. The sensor input and power outputlines as well as the power source line, all have the potential to couple or link the control circuit to a noise source.Depending on its intensity, noise can be coupled to the sensor circuitby any one or combination of the following ways: A quick review shows electrical noisecan enter the sensor circuit through different paths:1.Controller output signal lines 2.Power input lines3.Radiation (least likely to be aproblem)The sensitivity or susceptibility tonoisecoupling will be different among the three paths and may even vary on the same path, dependingon the type of electrical noise and its intensity.Following simple wiring techniqueswillgreatly decrease the sensor circuit’s sensitivity to noise.•Physical separation and wire rout-ing must be given carefulconsideration in planning thelayout. Ac power supply linesshould be bundled together and kept physically separate fromsensor signal lines. If lines mustcross, do so at right angles.•Another important practice is tolook at the system layout and identify electrical noise sourcessuch as solenoids, relay contacts, or motors, and where they arephysically located. Then use asmuch caution as possible to routethe sensor lead wires away fromthese noise sources. •Whenever possible, sensor signalleads should be run unbroken from sensor to the control.•Shielded cables should be usedfor all signal lines to protect frommagnetic and electrostaticcoupling of noise. Some simple pointers are as follows:1. Connect the shield to thecontrol circuit common end only. Never leave the shieldunconnected at both ends. Neverconnect both ends of the shield to a common.2. If the shield is broken at aterminal and the line continues, the shield must be reconnected to maintain shield continuity.3. If the shield is used as a signalreturn (conductor), no electrostatic industrial shielding can beassumed. If this must be done, use a triaxed cable(electrostatically shielded coaxialcable).4. Twisted wire should be usedany time sensor circuit signalsmust travel over two feet, or whenthey are bundled in parallel with other wires. 26