Voltage Burden - Keithley 2520 User Manual

F-20 Measurement Considerations

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
voltage burden (V
input resistance (R
Figure F-17
Voltage burden
R
DUT
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
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
line ground. Experimentation is the best way to determine an acceptable arrangement. For
to see how voltage burden affects detector current measurements. Note that
) reduces the measured current (I
B
).
IN
Model 2520
R
IN
I
V
M
B
I
V
DUT
S
V
+ V
S
B
R
+ R
DUT
IN
Figure F-19. Note that only a single instrument is connected directly to power
Model 2520 User's Manual
) due to the effects of the ammeter
M
Figure F-18 intro-