Yokogawa DA100 User Manual
Page 57
IM DA100-01E
2-36
• Shielded and twisted pair (prevention of electromagnetic coupling): an increase of
impedance
If it is difficult to keep the noise source away from the measuring instrument due to space
limitations, the use of a shielded twisted pair is effective.
· electrostatic coupling can be completely cut off by shielding;
· for a magnetic field, shielding with a magnetic material (iron, permalloy, etc.) can be employed.
However, there are many restrictions on this use and perfect shielding is impossible. Therefore,
use of a twisted pair is preferable.
Voltage e
m
induced by the coil is proportional to the area of the coil.
=> The smaller the area of the coil becomes, the smaller the noise becomes.
em
Magnetic field
Φ
Coil
If the directions of coils 1 and 2 are reversed by twisting, as shown, if the areas of the two coils
are equal, the induced voltages of the coils offset each other and total induced voltage em
becomes zero.
em
coil 1
coil 2
The above two principles are combined as a twisted pair.
Even though a shielded twisted pair is used, a proper grounding method is still important.
Connected
equipment
Signal
source
twisted pair
shield
If the signal source is not grounded
connected
equipment
Power
source
twisted pair
shield
Ground the signal cable shields collectively but separately from the power line ground. If the
separation of grounds is impossible, use the guard terminal.
2.9 Countering Noise