Appx.
9
Noise Countermeasures
Electromagnetic coupling from a high-current circuit
High-current circuits give off a magnetic field, and even larger magnetic fields may be generated by
transformers or choke coils with a large number of turns. The voltage induced by a magnetic field 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
2
7
7
0
N
V
75
.
0
m
1
.
0
1 A
0.001 m
Hz
60
10
4
10
4
2
d
d
d
d
µ
π
π
π
µ
φ
=
⋅
⋅
⋅
⋅
=
⋅
=
=
=
−
−
r
fI
r
IS
t
t
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 fluorescent 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.
索
索
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