Keithley 6482, Справочное руководство, Страница: 328 / 350

6482-901-01 Rev. A / August 2012 Return to
Section Topics
D-5
Model 6482 Dual-Channel Picoammeter / Voltage Source Reference Manual Appendix D: Measurement Considerations
• Use low noise cables. These cables are specially designed to minimize charge generation
and use graphite to reduce friction.
• Use the shortest cables possible, and secure them (for example, taping or tying) to a non-
vibrating surface to keep them from moving.
Piezoelectric and stored charge effects
Piezoelectric currents are generated when mechanical stress is applied to certain insulating
materials (for example, crystalline). In some plastics, pockets of stored charge cause the material
to behave in a similar manner.
When building test fixtures, choose good insulating materials and make connecting structures as
rigid as possible. Make sure there are no mechanical stresses on the insulators.
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 molecules 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 measurements. 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 D-1 to see how voltage burden affects current measurements. Assume V
S
is set to
output 5mV and R
L
is 5 k
Ω
. An ideal ammeter with zero voltage burden would measure the current
source as follows:
I
M
= V
S
/R
S
= 5 mV/5 k
Ω
= 1
In practice however, every ammeter has a voltage burden. If the voltage burden (V
B
) is 1 mV, the
current will be measured as follows:
I
M
= (V
S
+ V
B
) / R
S
= (5 mV + 1 mV) / 5 k
Ω
= 1.2
The 1 mV voltage burden caused a 20% measurement error.