Precision AC Current Shunt Set
Theory of Operation
23
Table 11. Other Error Sources and Solutions
Potential Error Source
Solution
Is the current stable, i.e., the same for both measurements?
Is the loading effect of the measuring device the same for each
measurement? It will not be if the Reference Shunt and the UUT
have significantly different resistance values resulting in a change of
the current in the circuit between measurements. The value of the
source resistance (Rs in Figure 5) influences the magnitude of the
error.
How significant is the difference between different measurement
devices? Switching would allow the same device to measure both the
UUT and the Reference Shunt, but switch impedance repeatability
may introduce more problems than it solves.
Careful selection of equipment.
All Current Shunts experience I
2
R self-heating to some extent. The
temperature coefficient of the Current Shunt causes a change in its
resistance value.
Careful selection of equipment.
Allow all elements in the system
to stabilize at normal operating
temperature before
measurements are started.
How will DC zero errors be compensated?
How will the measurement overcome thermal emf?
Take the average of positive and
negative DC measurements (DC
reversal).
AC Considerations
Most of the DC considerations also apply to AC measurements. Figure 6 shows the
measurement device input impedance as a resistance and capacitance to indicate that
impedance changes with frequency. The low input resistance value of SJTVC (160
Ω
) is
beneficial in this respect; it swamps the capacitance. Devices with active input circuits
generally have high input resistance and capacitance becomes more influential on
frequency flatness. Tables 4, 5, and 6 provide typical values and errors for Fluke 5790A
and 792A.
Rs
UUT
Shunt
Ref Std
Shunt
I
Gu
Gu
C
Gu
C
Gu
V Source
Lo
Hi
Lo
Hi
Zc
feh016.eps
Figure 6. Sources of Error in AC Measurements
Summary of Contents for A40B
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