F-6 Measurement Considerations Models 2500 and 2502 User’s Manual
Dielectric absorption
Dielectric absorption in an insulator can occur when a voltage across that insulator causes
positive and negative charges within the insulator to polarize because various polar mole-
cules relax at different rates. When the voltage is removed, the separated charges generate
a decaying current through circuits connected to the insulator as they recombine.
To minimize the effects of dielectric absorption on current measurements, avoid applying
voltages greater than a few volts to insulators being used for sensitive current measure-
ments. In cases where this practice is unavoidable, it may take minutes or even hours in
some cases for the current caused by dielectric absorption to dissipate.
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. If the voltage burden is large in relation to the
voltage of the measured circuit, then significant measurement errors will occur.
Refer to Figure F-1 to see how voltage burden affects current measurements. Assume V
S
is
set to output 5mV and R
L
is 5kΩ. An ideal ammeter with zero voltage burden would mea-
sure the current source as follows:
I
M
= V
S
/R
S
= 5mV/5kΩ
= 1
In practice however, every ammeter has a voltage burden. If the voltage burden (V
B
) is
lmV, the current will be measured as follows:
I
M
= (V
S
+
V
B
)
/ R
S
= (5mV + 1mV) / 5kΩ
= 1.2
The lmV voltage burden caused a 20% measurement error.
Figure F-1
Voltage burden
R
L
I
L
= <1mV
Model 2500
I
M
V
S
V
B
I
M
=
V
S
–
V
B
R
L
Test Equipment Depot - 800.517.8431 - 99 Washington Street Melrose, MA 02176
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