Fluke A40B, Instruction Manual

Page: 33 / 42

Precision AC Current Shunt Set
Theory of Operation
25
The TVC assembly comprises a 5 mA SJTVC and a 70
Ω
precision series resistor to drop
the 0.8 V output of the Reference Standard shunt to the 0.45 V maximum of the SJTVC.
The components of the TVC assembly are packed in expanded polystyrene in a metal box
to provide thermal stability. Construction is coaxial. The TVC assembly input connector
is an N-type plug (male) which mates directly with the Reference Standard output
connector.
Recommended Tools and Equipment
Table 12 provides a list of recommended equipment for using the Fluke Calibration
Method to calibrate the Current Shunts. When using substitutes for the recommended
models, make sure they meet or exceed the minimum use specifications shown in the
table.
Table 12. List of Recommended Equipment
Nomenclature Recommended Model Minimum Use Specifications
Multifunction Calibrator Fluke 5720A 0 to 10 volts, DC to 100 kHz
Transconductance Amplifier Clarke-Hess 8100 100 A, 100 kHz
Precision AC Current Shunt Set A40B Calibrated at a National Measurement
Institute or a Fluke Service Center
TVC Assembly (two required) A UHF, 5 mA Single Junction
thermocouple device or
thermal voltage convertor
such as the Fluke Model
A55, Ballantine 1394A, or
Measuretech EL1200; A
70
Ω
resistor.
0.8V, 5 mA DC to 100 kHz
Nanovoltmeter (two required) Keithley 2182A 10 mV DC, 1 nV resolution
System Calibration for AC-DC Difference
Periodic system calibration is required to compensate for the gain difference between the
two TVC assemblies. Figure 8 shows how this is achieved.
Keithley 21 8 2A
Digital
N ano v oltmeter 1
R
T V C assy
SJT V C
Fl u ke
5720A
Keithley 21 8 2A
Digital
N ano v oltmeter 2
R
T V C assy
SJT V C
feh020.eps
Figure 8. Calibrating Gain Difference
The Fluke 5720A produces an output of 0.8 V. The relative AC-DC difference of the
SJTC assemblies is measured and applied as a correction.