Fluke A40B, Instruction Manual

Page: 35 / 42

Precision AC Current Shunt Set
Theory of Operation
27
UUT AC-DC Difference Measurement Sequence and Calculations
In order to minimise the effects of drift, the measurement sequence is symmetrical in the
form AC1, +DC, -DC, AC2. The ±DC measurements cancel thermal emf and zero offset.
The two AC measurements compensate for drift during the measurement. Figure 10
shows the sequence where:
V
DCUUT
= average of magnitude of VDC
+UUT
and V
DC-UUT
V
DCSTD
= average of magnitude of VDC
+STD
and V
DC-STD
V
ACUUT
= average of V
ACUUT -1
and V
ACUUT -2
V
ACSTD
= average of V
ASTDT -1
and V
ACSTD -2
Apply AC signal
Initial Delay
V
ACUUT_1
V
ACSTD_1
Apply DC+ signal
Delay
Apply DC- signal
Delay
Apply AC signal
Delay
V
DCUUT
V
DCSTD
V
ACUUT
V
ACSTD
Calculations
Start
End
Simultaneous
measurement of
UUT and Std
V
DC+STD
V
DC+UUT
Simultaneous
measurement of
UUT and Std
V
DC-UUT
V
DC-STD
Simultaneous
measurement of
UUT and Std
V
ACUUT_2
V
ACSTD_2
Simultaneous
measurement of
UUT and Std
To overcome short term DC noise in the Clarke Hess 8100 the
output of the reference and UUT shunts are measured
simultaneously.
feh024.eps
Figure 10. AC-DC Difference Sequence