3-9
IM 253710-01E
Before Starting Measurements
3
Effects of Stray Capacitance
The effects of stray capacitance on the measurement accuracy can be minimized by
connecting the current input terminal of the instrument to the side that is close to the
earth potential of the power source (SOURCE)
The internal structure of the instrument is as follows.
The voltage and current measurement circuits are each enclosed in shielded cases.
These shielded cases are placed inside the outer case. The shielded case of the voltage
measurement circuit is connected to the
±
terminal of the voltage input terminal and the
shielded case of the current measurement circuit is connected to the
±
terminal of the
current input terminal.
Because the outer case is insulated from the shielded case, stray capacitance Cs of
approximately 100 pF exists. The current generated by this stray capacitance, Cs, will
cause errors.
U
±
I
±
Cs
Cs
Shielded case of the
voltage measurement circuit
Outer case
Earth
Shielded case of the
current measurement circuit
For example, we will consider the case when one side of the power source and the outer
case are grounded.
In this case, two current flows can be considered, load current i
L
and the current that
flows through the stray capacitance i
Cs
. i
L
flows through the current measurement circuit,
then through the load, and returns to the power source (shown with a dotted line). i
Cs
flows through the current measurement source, through the stray capacitance, and then
through the earth ground of the outer case, and returns to the power source (shown with
a dot-dash line).
Therefore, the current measurement circuit ends up measuring the sum of i
L
and i
Cs
even
though it wants to measure only i
L
. i
Cs
is the amount of error in this case. If the voltage
applied to Cs is V
Cs
(common mode voltage), then i
Cs
can be found using the following
equation. Because the phase of i
Cs
is ahead of the voltage by 90
°
the effects of i
Cs
on
the measurement accuracy increases as the power factor gets smaller.
i
Cs
= V
Cs
×
2
π
f
×
Cs
SOURCE
LOAD
Cs
i
L
i
L
i
Cs
±
±
I
U
i
Cs
i
L
When measuring high frequencies as in this instrument, this error, i
Cs
, cannot be ignored.
By connecting the current input terminal of the instrument to the side that is close to the
earth potential of the power source (SOURCE), the
±
terminal of the current
measurement circuit of the instrument approaches the earth potential. Thus, V
Cs
becomes approximately zero and very little i
Cs
flows. As a result, the effect on the
measurement accuracy is minimized.
3.5 For Making Accurate Measurements