D-2
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6482-901-01 Rev. A / August 2012
Appendix D: Measurement Considerations
Model 6482 Dual-Channel Picoammeter / Voltage Source Reference Manual
NOTE
This appendix summarizes considerations to make accurate measurements using the
Model 6482. For comprehensive information on these and additional measurement
considerations, refer to the Low Level Measurements handbook, which is available from
Keithley.
Low current measurements
Low current measurements are subject to a number of error sources that can have a serious
impact on measurement accuracy. These aspects are covered in the following paragraphs.
Leakage currents
Leakage currents are generated by high resistance paths between the measurement circuit and
nearby voltage sources. These currents can considerably degrade the accuracy of low current
measurements.
One way to reduce leakage currents is to use good quality insulators when building the test circuit.
Some good quality insulators include Teflon, polyethylene, and sapphire. Avoid materials such as
phenolics and nylon.
Humidity may also degrade low current measurements. The amount of water an insulator absorbs
will vary depending on the insulator. It is best to choose an insulator on which water vapor does
not readily form a continuous film. Sometimes this is unavoidable if the material being measured
absorbs water easily, so it is best to make the measurements in an environmentally controlled
room. In some cases, an insulator may have ionic contaminants and, especially in high humidity, a
spurious current may be generated.
Noise and source impedance
Noise can seriously affect sensitive current measurements. This topic discusses how DUT (device
under test) resistance and capacitance affect noise performance.
DUT resistance
The resistance of the DUT will affect the noise performance of the ammeters. As the DUT
resistance is reduced, the noise gain of the ammeter will increase. Noise gain can be given by the
following equation:
Output V
NOISE
= Input V
NOISE
(1 + R
F
/R
DUT
)
where;
•
Output V
NOISE
is the noise seen at the output of the ammeter.
•
Input V
NOISE
is the noise seen at the input of the ammeter.
•
R
F
is the internal feedback resistance for the ammeter.
•
R
DUT
is the resistance of the DUT.
Note that as R
DUT
decreases in value, the output noise increases. For example, when R
F
= R
DUT
,
the input noise is multiplied by a factor of two. Since decreasing the source resistance can have a
detrimental effect on noise performance, there are usually minimum recommended source
resistance values based on measurement range.
summarizes minimum recommended
source resistance values for various measurement ranges for the Model 6482 ammeters. Note
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