A40B
Instruction Manual
24
Common-mode current can be very significant in Current Shunt measurements especially
when one or more of the voltage measuring devices is mains (line) powered. The main
cause of common-mode current is capacitance between the measurement circuits and
ground. Careful guarding can reduce, but not eliminate, the effect. In Figure 6. Gu
represents instrument guards and C
Gu
the capacitance to ground. Notice that C
Gu
in the
lower measurement circuit is almost shorted out by the Gu / ground connections. The
voltage drop caused by connection impedances means that Gu in the lower circuit is not
exactly at ground and the difference changes with frequency. As a result, common-mode
current will cause small UUT measurement errors. Common-mode current errors usually
increase with frequency.
The errors caused by common-mode current in the Reference Standard measurement
circuit are potentially much greater. Zc in Figure 6 is the impedance of the connection
between the shunts. It can immediately be seen that the Gu / ground potential difference
in the upper circuit is higher so more common-mode current flows in C
Gu
of the
Reference Standard measurement circuit than in the UUT circuit. Note the connection of
the detector in the upper circuit. In this configuration C
Gu
is driven by the source current
reducing the effect of common-mode errors associated with the Reference Standard
shunt. The voltage across the UUT, Zc and the Reference Standard will increase with
frequency because of inductance. This compounds the frequency dependence of
common-mode current errors. Careful design of this connection to minimize inductance
is essential.
Common-mode chokes are often used in the input of measuring devices to reduce
common-mode effects. However, care must be taken to avoid resonances with C
Gu
within
the frequency range of interest.
The Fluke Calibration Method
The Fluke calibration method makes automated AC-DC difference and DC Resistance
measurements. See Figure 7. The voltage outputs from the reference and UUT shunts are
each detected with a 0.8 V, 5 mA SJTVC and a Keithley 2182A nanovoltmeter. A single
manual intervention during the calibration disconnects the TVC assemblies and connects
the shunt outputs to the nanovoltmeter secondary inputs. A comparative measurement is
made to calibrate resistance at nominal current.
Clarke-Hess model
8
100
Keithley 21
8
2A
Digital
N
ano
v
oltmeter
Rs
UUT
Sh
u
nt
Ref Std
Sh
u
nt
I
Fl
u
ke
5720A
R
T
V
C assy
SJT
V
C
Keithley 21
8
2A
Digital
N
ano
v
oltmeter
R
T
V
C assy
SJT
V
C
feh018.eps
Figure 7. The Fluke Calibration System
Summary of Contents for A40B
Page 6: ...A40B Instruction Manual iv...
Page 8: ...A40B Instruction Manual vi...
