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Shielding Instrumentation and Thermocouple Cable ã Thermometricscorp.com |
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Guidelines for shielding, grounding and physical separation from power circuits are provided. The general rules set forth are guidelines only and should be tempered by specific manufacturer's recommendations, particularly for computer applications.
Definitions
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a) Normal mode noise (transverse). The noise appearing between the signal wires caused by one of the following;
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1) Electrostatic induction due to distributed capacitance between the signal wires and the surroundings.
2) Electromagnetic induction due to magnetic, fields linking with the signal wires.
3) Junction or thermal potentials due to improper connections. |
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b)
Common mode noise (longitudinal). The noise appearing between each signal wire and ground caused by: |
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1) Incorrect location of signal; ground.
2 ) Multiple grounding of shield. |
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c )
Cross talk. The noise caused by AC and pulse type signals in adjacent circuits. |
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Recommendations for Noise Reduction |
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a ) Electrostatic normal mode noise can best be minimized by shielding.
b ) Electromagnetic normal mode noise can effectively be reduced by twisting signal conductor pairs, and physical separation between power and instrumentation cables.*
c ) Common mode noise caused by ground potential differences can best be minimized by grounding a shield at only one point and by the correct location of that ground point.
d ) Cross talk can best be reduced by using cables with twisted pair conductors and individually insulated shields over each pair.*
*The length and direction(s) of lay (right or left hand twists per inch) may depend on the cable design and are optimized in relation to performance requirements and good construction practices. |
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Shielding Recommendations |
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a ) Aluminum or copper backed tape, 100% coverage, with a copper drain wire (tinned copper should be used with aluminum) is a more effective electrostatic shield than a braid or served wire armor.
b ) At a minimum, low-level analog signal cables should be made up of twisted pairs with an overall shield.
c ) Digital signal cables and pulse type circuits should be wired with individually twisted and shielded pairs. In multipair cables, the shields should be isolated and an overall shield should also be provided.
d ) Except where specific reasons dictate otherwise, cable shields should be electrically continuous. When two lengths of shielded cable are connected together at a terminal block, a point on the terminal block should be used for connecting the shields.
e ) The shield of each cable should be covered with an insulating tape or jacket in order to prevent stray and multiple grounds to the shield. |
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Grounding Recommendations |
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a ) All shields should be grounded.
b ) Shields should be grounded at only one point.
c ) Digital signal circuits should not be grounded at any point external to the power supply.
d ) The shields on grounded as well as ungrounded thermocouple circuits should be grounded at the thermocouple well.
e ) Multipair cables used with thermocouples must have individual isolated shields so that each shield may be maintained at the particular couple ground potential.
f ) The low or negative potential side of a signal pair should be grounded at the same point where the shield is grounded. Where a common power supply is used, the low side of each signal and its shield should be grounded at the power supply. |
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ã Thermometricscorp.com |
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