Improving The Measurement Accuracy - YOKOGAWA WT110 User Manual

3.10 Improving the Measurement Accuracy

Recommended Wiring Method
3-16
The instrument is designed so that voltage input impedance is high and current input impedance
is low to reduce the effect of power loss on measurement accuracy.
Voltage input impedance : Approx. 2MΩ (all ranges), with a capacitance of appox. 15pF
Current input impedance : Approx. 6mΩ+0.1µH (all ranges)
From the explanation given below, it can be understood that the effect of power loss on
measurement accuracy can be reduced by wiring according to the load resistance.
SOURCE
V
±
A
±
Input terminal
(ELEMENT)
In the above diagram, the voltage measurement circuit is connected to the load side. The effects
of power loss on measurement accuracy are explained below. For simplification, it is assumed
that a DC power source and resistive load are used. The current measurement circuit measures
the sum of the current iL that flows to the load (object being measured) and the current iV that
flows to the voltage measurement circuit. This means that the current iV is erroneous since the
current to be measured is iL.
Since the input impedance of the voltage measurement circuit is high (appox. 2MΩ), and even if
the input voltage is 600V iV becomes approx. 0.3mA (=600V/2MΩ). If the instrumental error is
assumed to be lower than 0.1%, the measured current (iL) will be 300mA or higher (load
resistance is 2kΩ or lower). If the input voltage is 10V, iL is 5mA or higher. The relationship
between the input voltage and the measured current in cases where instrumental error is within
0.1% and 0.01% is given below as a reference.
0
Measured
100
voltage(V)
200
300
400
500
600
0.1% effect
In many cases the recommended wiring method is suitable. For instance, when input voltage
and current are 100V and 5A, iV is 0.05mA (=100V/2MΩ), therefore the effect on measurement
accuracy is 0.001% (=0.05mA/5A), which is low.
On the other hand, measurement accuracy is significantly affected when the measured current is
low (i.e. high load resistance). In this case, make the connections as follows so that the current
measurement circuit is located on the load side. The voltage measurement circuit measures the
sum of the voltage drop eL at the load and eA at the current measurement circuit, therefore eA
is erroneous. However, the effect of this error is small since the input impedance of the current
measurement circuit is low. For instance, if the load resistance is 600Ω, the input impedance is
approx. 6mΩ, therefore the error in measurement is approx. 0.001% (=eA/(eL+eA)), which is
low
P
SOURCE
V
±
±
This instrument
connected in parallel
LOAD
SOURCE
Measured current(A)
1 2 3 4
Effect decreases
0.01% effect
LOAD
e
L
A
Q
e
A
V
LOAD
i
V
±
i
L
±
A
This instrument
5
IM 253401-01E