A40B
Instruction Manual
14
Output Voltage Measurement – Loading Effect
The published specifications for a Current Shunt represent its performance under ideal
conditions. In practical use, placing the input of the voltage measurement device in
parallel with the Current Shunt introduces an additional impedance (loading effect) which
will result in a measurement error. See Figure 3. The 1 mA boxed Current Shunt has less
than 8 m
Ω
output resistance, so resistive loading effects are negligible. The capacitive
loading effect is less than 15 ppm per 100 pF at 100 kHz. For the non-active Current
Shunts, the loading effect becomes more significant as the resistance value of the Current
Shunt increases, that is, as the nominal current value decreases. For the most accurate
measurements, the error due to this loading effect must be calculated and used as a
measurement correction.
Z Connect
Z Connect
Current
Input
Voltage
output
Shunt Resistor
Device Input Z
feh004.eps
Figure 3. Loading Effect of the Measurement Device
Output Voltage Measurement Using a Fluke 5790A
The typical input impedance of a 5790A (input 1 or 2) is >10 M
Ω
(dc) in parallel with
70 pF. The 10 M
Ω
in parallel with the shunt resistance has a small effect on the lowest
current value Current Shunts at DC. The 5790A impedance typically decreases to
approximately 1.5 M
Ω
in parallel with 70 pF at 100 kHz, and, at this frequency, the
loading effect is more pronounced. For the lower value shunts, the loading error is
comparable with or exceeds the shunt error when used to measure current directly.
Table 4 shows the loading effect of the 5790A on Current Shunts in the range of 10 mA
to 200 mA.
Table 4. Typical Loading Effect of a 5790A, 2.2 V Range
DC
1 kHz
10 kHz
30 kHz
100 kHz
Current
(mA)
Loading
Error
(ppm)
Loading
Error
(ppm)
AC-DC
Diff.
(ppm)
Loading
Error
(ppm)
AC-DC
Diff.
(ppm)
Loading
Error
(ppm)
AC-DC
Diff.
(ppm)
Loading
Error
(ppm)
AC-DC
Diff.
(ppm)
1
0 0 0 -1
+1 -3
+3
-11
+11
10
-6 -6 0 -8 +2 -16
+10 -56
+49
20
-3 -3 0 -4 +1 -8 +5 -26
+23
50
-1 -1 0 -2 0 -3 +2 -10 +9
100
-1 -1 0 -1 0 -2 +1 -5 +4
200
0 0 0 0 0 -1 0 -2
+2
Summary of Contents for A40B
Page 6: ...A40B Instruction Manual iv...
Page 8: ...A40B Instruction Manual vi...
