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F-20 Measurement Considerations Model 2520 User’s Manual
Voltage burden
The input resistance of the ammeter causes a small voltage drop across the input terminals.
This voltage is known as the voltage burden and it can cause measurement errors. Refer to
Figure F-17 to see how voltage burden affects detector current measurements. Note that
voltage burden (V
B
) reduces the measured current (I
M
) due to the effects of the ammeter
input resistance (R
IN
).
Figure F-17
Voltage burden
General measurement considerations
The following measurement considerations apply to all precision measurements.
Ground loops
Ground loops that occur in multiple-instrument test setups can create error signals that
cause erratic or erroneous measurements. The configuration shown in
Figure F-18 intro-
duces errors in two ways. Large ground currents flowing in one of the wires will encounter
small resistances, either in the wires, or at the connecting points. This small resistance
results in voltage drops that can affect the measurement. Even if the ground loop currents
are small, magnetic flux cutting across the large loops formed by the ground leads can
induce sufficient voltages to disturb sensitive measurements.
To prevent ground loops, instruments should be connected to ground at only a single point,
as shown in
Figure F-19 . Note that only a single instrument is connected directly to power
line ground. Experimentation is the best way to determine an acceptable arrangement. For
R
DUT
I
DUT
Model 2520
I
M
V
S
V
B
I
M
=
V
S
+ V
B
R
DUT
+ R
IN
R
IN