Hioki DM7275-01 - Page 189

Appx.9
Noise Countermeasures
Electromagnetic coupling from a high-current circuit
High-current circuits give off a magnetic eld, and even larger magnetic elds may be generated by
transformers or choke coils with a large number of turns. The voltage induced by a magnetic eld is
affected by distance and area (see Figure 3). A voltage of about 0.75 μV will be generated in a 10
cm
2
loop positioned 10 cm away from a 1 A commercial power supply.
RMS
RMS
27
7
0
N
V75.0
m1.0
1 A0.001 mHz60104
104
2d
d
d
d
µ
π
π
π
µφ
=
⋅⋅⋅⋅
=
⋅
=






==
−
−
r
fI
r
IS
tt
v
To counter the effects of electromagnetic coupling, it is effective to keep voltage detection wires
away from lines that are generating noise and to twist them together (see Figure 4).
V
Low
High
v
N
V
V+v
N
V
Low
High
V
S
Low
Noise voltage
Voltage
Voltage
Looparea
Magnetic
flux
φ
Figure 3. Noise Coupling from
a High-current Circuit
Figure 4. Noise Countermeasure:
Twisting Wires Together
Twisting wires together
Ifinducednoiseiscausedbythecommercialpowersupply
Induced noise caused by commercial power supplies can come not only from commercial power
lines and power outlets, but also from uorescent lights and household appliances. Such noise
depends on the commercial power supply frequency and occurs at a frequency of 50 Hz or 60 Hz.
One method typically used to reduce the effects of noise caused by commercial power supplies is
to set the integration time to a whole-number multiple of the power supply cycle (see Figure 5).
Figure 5. Averaging of Noise through Integration
Measurementsignalwithsuperposedpowersupplynoise
Ideal measurement signal (DC)
Integration time
Magnetic
flux
φ
10
9
8
7
6
5
4
3
2
1
Appx.
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