Keithley 6521, Инструкция по эксплуатации, Страница: 36 / 65

Operation
3-8
Regardless of the source of the offsets, they can be nulled us-
ing the electrometer REL feature. See the Model 6517 User’s
Manual for more information on using relative.
Triboelectric effects
Triboelectric currents are generated by charges created be-
tween a conductor and insulator by friction. In this situation,
free electrons rub off the conductor and create a charge im-
balance that causes the current to fl w. A typical example
would be the electrical currents generated in a coax or triax
cable by fl xing that cable or otherwise subjecting it to stress.
Low-noise cables are constructed with a special graphite
coating under the shield(s) to minimize friction and provide
a conducting path to minimize charge generation. Keithley
triax cables and special low-noise coax cables are construct-
ed in this manner; such cables should be used for any low-
level measurements made using the Model 6521/22 Scanner
Card. Note the conventional cable such as RG-58 is not rec-
ommended for use with the Model 6521 because of high
noise currents.
Even low-noise cables generate some currents when subject-
ed to stress. For that reason, the following precautions should
be taken to minimize unwanted currents that might be gener-
ated by cables:
• Keep all cables as short as possible.
• Keep cable temperature variations to a minimum.
• Tie down or tape all connection cables to a rigid pole or
fixture
• Do not bend or fl x cables during sensitive
measurements.
• Keep vibration sources such as motors and pumps well
away from connecting cables.
Dielectric absorption
Dielectric absorption in a triax or coax connector insulator
can occur when a voltage applied to the insulator causes pos-
itive and negative charges within that insulator to separate.
When the voltage is removed, the separated charges generate
a decaying current through circuitry connected to the scanner
card.
To minimize the effects of dielectric absorption, avoid apply-
ing more than a few volts to the input or output connectors of
a scanner card to be used for sensitive current measurements.
In cases where this practice is unavoidable, it may take min-
utes or even hours in some cases for currents caused by di-
electric absorption to dissipate.
3.6.2 Path isolation
The path isolation is simply the equivalent impedance be-
tween any two test paths in a measurement system. Ideally,
the path isolation should be infinite, but the actual resistance
and distributed capacitance of cables and connectors results
in less than infinite path isolation values for these devices.
The capacitive component of path isolation impedance is
generally fi ed by design, but the resistive component can be
reduced by environmental and other factors, as we will now
discuss.
Path isolation resistance forms a signal path that is in parallel
with the equivalent resistance of the DUT, as shown in Fig-
ure 3-1. For low-to-medium device resistance values, path
isolation resistance is seldom a consideration; however, it
can seriously degrade measurement accuracy when testing
high-impedance devices. The voltage measured across such
a device, for example, can be substantially attenuated by the
voltage divider action of the device source resistance and
path isolation resistance, as shown in Figure 3-2.
Another phenomenon negatively affected by lower path re-
sistance values are leakage currents that can be generated
through these resistances by voltage sources in the system.
Such leakage currents can, of course, seriously affect low-
level current measurement accuracy. Thus, it is imperative
that the path isolation resistance be as high as possible in or-
der to minimize these effects.
R
E
DUT
DUT
R
PATH
V R
IN
DUT
Scanner
Card
Model 6517
= Source Resistance of DUT
= Source EMF of DUT
= Path Isolation Resistance
= Input Resistance of Model 6517
R
DUT
E
DUT
R
PATH
R
IN
Figure 3-1
Path isolation resistance