Hioki LR8431, Instruction Manual

Page: 172 / 204

Appendix 8 Concerning Noise Countermeasures
A
14
Equivalent Circuit of Noise Introduction Path
Common mode noise results from the interposition of ground impedance between
the ground point of the device being tested and the ground point of the instrument,
and from capacitor coupling between the ground wire and noise source.
Common mode noise is converted to normal mode voltage (Enm) that is added to
the "+" and "-" input terminals of the instrument as a result of the noise current (i1)
and (i2) that flows to the coupling impedance (Z1) and (Z2) between the "+" and "-"
input terminals of the instrument and ground. Because common mode noise is gen-
erated between the input pins, it has a direct effect on the measurements.
Inverter or Commercial
Power Supply
Electromagnetic
Coupling
Signal Voltage
Normal Mode Voltage
Coupling Impedance
Instrument
Voltage Generated by Ground Current
(common mode voltage)
Ground

Resistance
Ground Impedance
Ground

Resistance
Leak current from other devices and ground current
flow due to difference in ground potential.
Electrostatic Coupling
Measurements are directly affected by normal mode voltages from electromagnetical-
ly induced noise resulting from electromagnetic coupling into looped instrument input
lines by AC magnetic fields produced by inverters and commercial power lines, as
well as the capacitive coupling that results from interline capacitance.