385
Noise Countermeasures
Common-mode noise countermeasures
Device under measurement
Measurement module
Thermocouple
Enm'
r
1
r
2
Vin
Z
1
Z
2
Z
3
iearth
Zearth
Ecm
Zearth Zearth
Coupled impedance
Electrostatic
induction noise
Inverter or commercial
power supply
Electromagnetic
induction noise
Voltage produced by earth current (common-mode voltage)
Ground line
Normal-mode voltage
Ground impedance
i
2
=0 i
1
= 0
Connect the chassis GND on the
signal side to a suitable ground with
adequately low grounding resistance.
Either connect to a ground line with low resistance or
bypass the grounding electrodes. This will limit the
common-mode voltage and reduce noise current (i
1
, i
2
).
The eect on measured values will be reduced since
normal-mode voltage is limited.
Blocking external noise
• Keep signal lines away from noise sources.
Keep input signal lines (thermocouples) away from wires that could serve as sources of noise
(power lines, etc.).
Alternatively, install devices so that wires are kept as far apart as possible, for example by
running wires through separate ducts.
• Use shielded twisted-pair wire.
One eective way to counteract common-mode noise is to use shielded twisted-pair wires for
input signal lines (thermocouples).
Twisted-pair wires prevent electromagnetic induction, while shielded wires prevent electrostatic
induction.
Ground shielded wires at the signal source.
For more information about shielded twisted-pair wires for thermocouples, contact the
thermocouple manufacturer.
Knowledge and Information
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