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Appendix 8 Mitigating Noise
A19
A
ppend
ix
(1) Effects of induced noise
Power cords, fluorescent lights, solenoid valves, computer displays, and other devices emit
large amounts of noise. Two sources of noise with the potential to affect resistance mea-
surement are:
1. Capacitive coupling from high-voltage lines
2. Electromagnetic coupling from high-current lines
Capacitive coupling from high-voltage lines
Current flowing from a high-voltage line is dominated by the coupled capacitance. As an
example, if a 100 V commercial power line and a wire used in resistance measurement are
subject to capacitive coupling of 1 pF, a current of about 38 nA will be induced.
When measuring a 1
Ω
resistor with 100 mA,
this effect is just 0.4 ppm and can be safely
ignored.
However, when measuring 1 M
Ω
with 10
μ
A,
the effect increases to 0.38%. In this way,
capacitive coupling from high-voltage lines
requires caution during high-resistance mea-
surement. It is effective to provide static shield-
ing for wires and measurement targets (see
Fig. 1).
Electromagnetic coupling from high-current lines
High-current lines emit a magnetic field. Transformers and choke coils with a large number
of turns emit an even stronger magnetic field. The voltage induced by the magnetic field is
affected by the distance and area. A loop of 10 cm
2
located 10 cm from a 1 A commercial
power line will generate a voltage of about 0.75
μ
V.
Appendix 8 Mitigating Noise
RM S RMS
nA 38 V 100 pF 1 60 2
= ⋅ ⋅ ⋅ = =
π
Z
V
I
Figure 1
. Static Shielding near
High-voltage Wires
Static shielding
Fluorescent light
RMS
RMS
2 7
7
0
V 75 . 0
m 1 . 0
A 1 m 001 . 0 Hz 60 10 4
10 4
2 d
d
d
d
μ
=
⋅ ⋅ ⋅ ⋅
=
⋅
=




= =
−
−
π
π
π
μ
φ
r
fI
r
IS
t t
v
f I
μ