App-2
IM WT310E-02EN
Effects of Stray Capacitance
The effects of stray capacitance on measurement accuracy can be minimized by connecting the
current input terminal of this instrument to the side of the power supply (SOURCE) that is closest to its
earth potential.
The internal structure of this instrument is explained below.
The voltage and current measurement circuits are each enclosed in shielded cases. These shielded
cases are contained within an outer case. The shielded case of the voltage measurement circuit is
connected to the positive and negative voltage input terminals, and the shielded case of the current
measurement circuit is connected to the positive and negative current input terminals.
Because the outer case is insulated from the shielded cases, there is stray capacitance, which is
expressed as Cs. Cs is approximately 40 pF. The current generated by stray capacitance Cs causes
errors.
V
C
Cs
Cs
Shielded case of the voltage
measurement circuit
Outer case
Grounding
Shielded case of the current
measurement circuit
±
±
As an example, we will consider the case when the outer case and one side of the power supply are
grounded.
In this case, there are two conceivable current flows, i
L
and i
C
s. i
L
is the load current, and i
C
s is the
current that flows through the stray capacitance. i
L
flows through the current measurement circuit, then
through the load, and returns to the power supply (shown with a dotted line). i
C
s flows through the
current measurement circuit, the stray capacitance, and the earth ground of the outer case, and then
returns to the power supply (shown with a dot-dash line).
Therefore, the current measurement circuit ends up measuring the sum of i
L
and i
C
s, even if the
objective is just to measure i
L
. Only i
C
s reduces measurement accuracy. If the voltage applied to Cs is
V
C
s (common mode voltage), i
C
s can be found using the equation shown below. Because the phase
of i
C
s is ahead of the voltage by 90°, the effect of i
C
s on the measurement accuracy increases as the
power factor gets smaller.
i
C
s = V
C
s × 2πf ×
C
s
SOURCE
LOAD
Cs
i
L
i
L
i
Cs
C
V
i
Cs
i
L
±
±
Appendix 1 How to Make Accurate Measurements
