Hioki DM7275 Series, Инструкция по эксплуатации

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Appx.
8
Noise Countermeasures
Appx. 4 Noise Countermeasures
Effects of induced noise
A significant amount of noise may be generated by components and devices such as power cords,
fluorescent 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 flowing 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:
RMS RMS N
nA 38 V 100 pF 1 60 2
= ⋅ ⋅ ⋅ = =
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).
RMS RMS N OUT DISPLAY
A 38 38 nA k 1
µ
+ = ⋅ Ω + = + =
V V i R V V
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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