Precision AC Current Shunt Set
Theory of Operation
21
Potential Voltage Measurement Devices
TVC
The system could use thermal voltage converters (TVC) in conjunction with a DC
voltmeter to measure the voltage output from a Current Shunt. The TVC produces a dc
output voltage proportional to the ac or dc voltage applied and is intrinsically flat across
the entire frequency range required for this application. The TVC can only be used to
compare two signals and does not provide an absolute measurement; hence its main use is
AC-DC difference measurements. There are various forms of TVC; Table 8 lists
advantages and disadvantages of the simplest: the single junction TVC (SJTVC).
Table 8. Single Junction Thermal Voltage Converters
Advantage Disadvantage
Frequency flatness errors are generally very small.
Slow to reach thermal stability
Input impedance is mainly resistive
Low input resistance (90
Ω
)
Very good output / input isolation. Negligible errors
due to common-mode current
Maximum input voltage is 0.45V. Appropriate
series resistor is required to increase input
voltage capability
Square law response offers poor amplitude
linearity
Nanovoltmeter or potentiometer /null detector
required to resolve mV output levels
Susceptible to small changes in local
environment (draughts, heat source)
Fluke 792A AC-DC Transfer Standard
The Fluke 792A can be used to make AC-DC difference measurements and has many
advantages over the SJTVC. See Table 9. It does, however, have one disadvantage that
reduces its usefulness for this application. Unlike the single junction TVC, the 792A is
subject to common-mode interference, particularly when used as the measurement device
for the Reference Standard shunt.
Table 9. Fluke 792A AC-DC Transfer Standard
Advantage Disadvantage
Frequency flatness errors are generally very small.
Common-mode errors
[1]
Solid state TVC with 2 V output.
Requires DC voltmeter to resolve output
Faster to settle than SJTVC
Input impedance varies with frequency
Smaller DC reversal error
Linear response
High input impedance
[1] Common-mode errors are the biggest source of error for mains powered devices.
Summary of Contents for A40B
Page 6: ...A40B Instruction Manual iv...
Page 8: ...A40B Instruction Manual vi...
