Circuit Loading Error - Fluke 8050A Instruction Manual

2. Connect the 8050A to the Voltage Standard as
follows: V
IkO/S
input connector to the
Voltage Standard Hi output, COMMON input
connector to the Voltage Standard low output.
3. Select the V function (either ac or dc,
depending upon the Voltage Standard used).
4. Adjust the Voltage Standard to output the
reference level calculated in step I. This value
should appear in the 8050A display.
5. Select the dB function.
6. Set the RELATIVE switch to ON. Decibel
measurements will now be referenced to the
circuit impedance.
248. Use the following procedure to obtain a correction
factor for referencing dB to other circuit impedances
without the use of a Voltage Standard:
I. Select the REF Z that is closest to the
impedance of the circuit to be measured.
2. Calculate the reference impedance correction
factor using the following equation.
Circuit Impedance
10 log REF Z
= Correction Factor
3. Add the correction factor to the measured
value.
2-49. CIRCUIT LOADING ERROR (VOLTAGE)
2-50. Circuit loading errors occur when voltage
measurements are taken on high impedance circuits. This
is because the DMM loads the source, thus changing the
operating voltage of the source. As long as the circuit
impedance (source impedance) is low compared to the
input impedance of the DMM this error may be
insignificant. For example, when measuring a circuit with
a source impedance of I kO or less, the error will be
~
.0I
%.
If the circuit loading error is significant, use the
appropriate formula contained in Figure 2-10 to calculate
the percentage of error.
2-51. Current Measurement
2-52. The following paragraphs contain additional
information on and measurement techniques for current
measurements.
OPERATION
RESISTANCE MEASUREMENT
WARNING
OPERATOR INJURY AND INSTRUMENT
DAMAGE MAY RESULT IF THE BACKUP
FUSE (F2) BLOWS WHEN CURRENT IS
BEING MEASURED FROM A VOLTAGE OF
GREATER THAN 800 VOLTS.
2-53. BURDEN VOLTAGE ERROR
2-54. When a multimeter is placed in series with a circuit
to measure current, the voltage drop of the multimeter
induces an error. This error is called the burden voltage.
The maximum full-scale burden voltages for the 8050A
are 0.3V for the four lowest ranges and 0.9V for the
highest range.
2-55. These voltage drops can affect the accuracy of the
current measurement if the current source is unregulated
and the resistance ofthe shunt and fuses ofthe multimeter
exceeds 1/1000 of the source resistance. If the multimeter
burden voltage is significant, the formula in Figure 2-11
can be used to calculate the burden voltage error.
2-56. Resistance Measurement
2-57. The following paragraphs contain additional
information on and measurement techniques for
resistance measurements.
1. DC VOLTAGE MEASUREMENTS
Loading Error in %::; 100 x Rs';- (Rs
+
10
7
)
Where: Rs::; Source resistance in ohms of
circuit being measured.
2. AC VOLTAGE MEASUREMENTS
First, determine input impedance, as follows:"
10
7
Zin ::;
Vl
+
{2
1f
F . Rin . Cin)2
Where: Zin::; effective input impedance
Rin::; 10
7
ohms
Cin ::; 100 x 10
-12
Farads
F ::; frequency in Hz
Then, determine source loading error as follows:"
Loading Error in
%
~
100 x
Where: Zs ~ source impedance
Zin input impedance (calculated)
"Vector algebra required
Figure 2-10. Circuit Loading Error
2-13