Hioki Super megohm SM7110 - Countermeasures Against Noise

Appx.4
Countermeasures Against Noise
Appx. 4 Countermeasures Against Noise
(1) E󰀨ects of induction noise
A lot of noise is generated from power cords, uorescent lights, solenoid valves, and computer displays. Below
are noise sources that may a󰀨ect resistance measurement.
1. Electrostatic coupling between a high-voltage line and a measurement lead
2. Magnetic coupling between a high-current line and a measurement lead
Electrostatic coupling between a high-voltage line and a measurement lead
Current that ows in from a high-voltage line is controlled by electrostatic capacitance coupled with a
measurement lead.
For example, if a 100 V commercial power line and a resistance measurement lead are electrostatically
coupled with a capacitance of 1 pF, a current of approximately 38 nA is induced.
I
V
Z
= = ⋅⋅
⋅=
2601 100 38
π
pF V nA
RMS RMS
If a 1
Ω
resistor is measured with a measurement
current of 100 mA, the e󰀨ect reaches to only 0.4
ppm of the measured value and may be ignored.
If a resistance of 1 M
Ω
is measured with a
measurement current of 10 μA, the e󰀨ect is only
0.38% to the measured value. For high resistance
measurement, care against electrostatic coupling
between a high-voltage line and a measurement
lead should be exercised. Shielding measurement
leads and objects to be measured electrostatically
is e󰀨ective (Figure 1).
R
X
A
Fluorescent light
Electrostatic shield
Figure 1 Electrostatic shield close to high-voltage wiring
Magnetic coupling between a high-current line and a measurement lead
A magnetic eld is generated from a high-current line. An even larger magnetic eld is emitted from
transformers and choke coils with a lot of turns. A voltage induced by the magnet eld is a󰀨ected by distance
and surface area. A loop with a area of 10 cm
2
placed 10 cm away from a commercial power line carrying a
current of 1A generates a voltage of 0.75 μV.
v
d
dt t
μ IS
= =






=
⋅⋅
=
⋅⋅ ⋅⋅
−
−
φ
d
dr
fI
r
0
2
π
π
π
410
410600001 1
7
72
Hz m. AA
m
μV
RMS
RMS
01
07 5
.
.=
If a 1 m
Ω
resistor is measured with a measurement
current of 1 A, the e󰀨ect is 0.07% of the measured
value. With high resistance measurements, there
is no issue because the detection voltage can be
increased easily.
Twisting each pair of voltage detection leads,
which are for resistance measurement, and noise
generating lines in addition to separating the pairs
from each other will e󰀨ectively decrease the impact
of magnetic coupling (Figure 2).
V R
X
V
X
A
I
X
Twist leads
Figure 2 Twist leads close to a high-current line