Hioki DM7275 Series - Appx. 4 Noise Countermeasures. Appx.8; X84; Effects of Induced Noise

Appx.8
Noise Countermeasures
Appx.4 Noise Countermeasures
Effects of induced noise
A signicant amount of noise may be generated by components and devices such as power cords,
uorescent lights, solenoid valves, and computer displays. The following phenomena may result in
noise that affects resistance measurement:
1. Capacitive coupling from high-voltage circuits
2. Electromagnetic coupling from high-current circuits
Capacitive coupling from high-voltage circuits
Current owing in from a high-voltage circuit is dominated by the coupled capacitance. As an
example, a current of about 38 nA will be induced when a 100 V commercial power line and wiring
used to measure resistance undergo capacitive coupling at 1 pF:
RMSRMSN
nA38V100pF1602 =⋅⋅⋅==
π
Z
V
i
The noise current is converted into the noise voltage
R
OUT
i
n
by the output resistance
R
OUT
. If the
output resistance is 1 k
Ω
, noise of 38 μV
RMS
will be superposed onto the detected voltage, causing
a change in the measured value (see Figure 1).
RMSRMSNOUTDISPLAY
A3838 nAk1
µ
+=⋅Ω+=+= VViRVV
Close to high-voltage circuits, it is effective to shield measurement cables and the measurement
target with a low-impedance line from the instrument (see Figure 2). The instrument’s LOW terminal
is a low-impedance line.
V
R
OUT
Low
High
V
Low
High
i
N
V
i
N
V+R
OUT
i
N
Figure 1. Noise Coupling from
a High-voltage Circuit
Figure 2. Noise Countermeasures
Using Shielding
Fluorescent light
Shield wire
Fluorescent light
Voltage
Output resistance
Static shielding
Noise current
Noise current
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