Tutorial 7
Keysight 34970A/34972A User’s Guide 287
Shunt Impedance
The insulation used for thermocouple wire and extension wire can be degraded by
high temperatures or corrosive atmospheres. These breakdowns appear as a
resistance in parallel with the thermocouple junction. This is especially apparent
in systems using a small gauge wire where the series resistance of the wire is high.
Shielding
Shielding reduces the effect of common mode noise on a thermocouple
measurement. Common mode noise is generated by sources such as power lines
and electrical motors. The noise is coupled to the unshielded thermocouple wires
through distributed capacitance. As the induced current flows to ground through
the internal DMM, voltage errors are generated along the distributed resistance of
the thermocouple wire. Adding a shield to the thermocouple wire will shunt the
common mode noise to earth ground and preserve the measurement.
Common mode noise can dramatically affect the internal DMM. A typical
thermocouple output is a few millivolts and a few millivolts of common mode
noise can overload the input to the internal DMM.
HI
LO
DMM
WITH SHIELD
WITHOUT SHIELD
Distributed
Distributed
Capacitance
Resistance
Power Line
DMM
LO
HI
Power Line
C
R
CC
C
C
C
C
C
C
R
R
R
R
R
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