Appendix 8 Concerning Noise Countermeasures
A13
Appendix
Mechanism of Noise Introduction into Thermocouple Temperature Mea-
surements
What are the sources of noise?
Within a factory, power is provided by large current flows at 50/60Hz. The main
loads are primarily L loads, such as motors and solenoids; in addition, large current
pulses are produced by capacitor input-type switching power supplies for device
such as inverters and high-frequency induction furnaces. Basic wave component
leak current, harmonic wave current, and other types of noise become mixed into the
flow from each ground point to ground lines.
What are the paths of noise propagation?
• Common mode voltage introduced between the ground points of the device being
tested and the instrument leaks to the input signal lines
• AC magnetic fields produced by current in power lines couple into loops in the
input signal lines
•
Coupling due to electrostatic capacitance between inpu
t signal lines and power
supply lines
What is common mode noise?
Noise that is generated between ground and the "+" and "-" input terminals of the
instrument.
What is normal mode noise?
Inter-line noise that is generated between the "+" and "-" input terminals of the instru-
ment.
Example of Connections Where Noise Can Affect Measurements
Appendix 8 Concerning Noise Countermea-
sures
When using a thermocouple to measure temperature, noise can affect measurements
unless both the device being tested and the instrument are grounded.
Although there is no problem if the instrument is running on battery power, the instru-
ment must be connected to ground if an AC adapter is being used.
Thermocouple
Device being
Tested
Instrument
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