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Noise Countermeasures
11.5 Noise Countermeasures
This section describes how to deal with a noisy environment.
Noise contamination mechanisms
Noise sources
Manufacturing plants use large currents at 50 Hz/60 Hz as a source of power. Many loads are
characterized by an inductive load, for example motors and solenoids. Equipment such as inverters
and high-frequency induction furnaces uses capacitor input-type switching power supplies that
carry large pulsed currents. Fundamental frequency component leakage currents, harmonic
currents, and other components ow together on ground lines.
Noise propagation routes
• Routes by which common-mode voltage between the measurement target and instrument ground
leaks to input signal lines
• Routes by which an alternating eld is coupled to a loop in the input signal line by the power
supply line current
• Routes created by coupling due to capacitance between the input signal lines and power supply
lines
Common-mode noise
Noise occurring between the instrument’s positive and negative input terminals and ground
Normal-mode noise
Line noise occurring between the instrument’s positive and negative input terminals
Example of a connection that is susceptible to the eects of noise
Device under measurement
Measurement module
r 1
r 2
Z
3
Z
1
Z
1
Thermocouple
V in
When measuring temperature with a thermocouple, the circuit will be susceptible to noise
unless both the device under measurement and the measurement module have been grounded.
This is not an issue when powering the measurement module with the battery, but ground the
measurement module if using the AC Adapter.
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