Field Wiring Primer

7/31/2019 Field Wiring Primer

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http://www.ni.com/


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Field Wiring and Noise ConsiderationsFor Analog Signals

Presented By National Instruments Field Sales Engineer

Travis McCarty

Cell: (832) [email protected]
http://www.ni.com/


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Agenda

Types of Signal Sources

Types of Measurement Systems

Measuring Grounded Signal Sources

Measuring Floating (Nonreferenced) Sources

Minimizing Noise Coupling in the Interconnects

Solving Noise Problems in Measurement Setups Signal Processing Techniques for Noise Reduction


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Types of Signal Sources

Ground-Referenced Floating (Nonreferenced)

Referenced to building system ground

Grounds may not all be at the same

potential ( 10 mV 200 mV)

Not referred to an absolute reference

Examples: batteries, battery powered

signal sources, thermocouples,

transformers, and isolation amplifiers


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Types of Measurement Systems

Differential Referenced


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Measuring Grounded Signal Sources

Use a differential measurement system


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Using a differential measurement system

Must ensure that the common-mode voltage level of the signal with respect to measurement

system ground remains in the common-mode input range of the measurement device

Bias resistors provide a DC path from the instrumentation amplifier inputs to the

instrumentation amplifier ground

NI typically recommends bias resistors between 10 k and 100 k for low impedance

sources


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Using a referenced measurement system


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First, identify the possible noise coupling

mechanisms that may be present:

Conductive

Capacitive

Inductive

Then, work to minimize the impact of each one


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Conductive Coupling

Results from sharing currents from different circuits in a

common impedance

May be minimized by breaking ground loops (if any) and

providing separated power and ground returns


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Avoiding Conductively Coupled Noise


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Capacitive Coupling

Results from time-varying electric fields in the vicinity of

the signal path

Combat by placing a shield between the capacitively

coupled conductors and grounding the shield at one end

The capacitive shield will provide another path for the

induced current so it is not carried in the signal circuit


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Minimizing Capacitively Coupled Noise


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Inductive Coupling

Results from time-varying magnetic fields in the area

enclosed by the signal circuit

Minimize this type of noise by using twisted-pair wiring

because it reduces the loop area in the signal circuit

Clamp cabling to avoid the motion of dangling wires


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Solving Noise Problems in Measurement Setups

Process of trial and elimination to locate the cause

of the interference problem

Test the DAQ system by itself [short Ch (+) and Ch (-) to

the analog input ground]

Test the cabling (replace signal source with something

else either a battery or resistor)

Test the transducer with a portable DAQ device


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Signal Processing Techniques for Noise Reduction

Time averaging

High or low pass filtering

Band pass filtering Notch filtering

Leverage Express VIs


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Field Wiring and Noise ConsiderationsFor Analog Signals

Read the white paper online at

http://zone.ni.com/devzone/cda/tut/p/id/3344

Travis McCarty

Cell: (832) 748-7017

[email protected]

For Further Information on the Topic of
http://zone.ni.com/devzone/cda/tut/p/id/3344http://zone.ni.com/devzone/cda/tut/p/id/3344http://www.ni.com/


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Field Wiring Primer