Keithley 6521, Инструкция по эксплуатации, Страница: 40 / 65

Operation
3-12
3.7.2 High-resistance measurements
Resistors with values of 1G Ω or more are often referred to as
high-megohm resistors. Such resistors often exhibit unusual
characteristics such as changes in resistance with applied
voltage. (Such change in resistance is known as the voltage
coefficient, which is g ven in percent/volt, or ppm/volt.)
As with many other component evaluation tests, high-meg-
ohm resistance tests are often best performed on an automat-
ed basis since testing usually involves multiple devices. Fig-
ure 3-7 demonstrates a test system using the Model 6517
Electrometer along with a Model 6521/22 Scanner Card to
perform automated high-resistance testing on up to 10 resis-
tors simultaneously. The test voltage is, of course, supplied
by the Model 6517 voltage source, while the electrometer
section measures the current through each resistor. The in-
strument then computes and displays the value of each resis-
tor automatically.
Since the resistance of such devices varies with applied volt-
age, all testing should be done at the same voltage for com-
parative analysis to be valid. Alternatively, the voltage
coefficient may be determined by measuring the resistance at
two different voltages and then computing the voltage coef-
ficient as foll ws:
Voltage Coefficient (%)
R
2
R
1
–
R
1
V
2
V
1
– ( )
-----------------------------
100
× =
Where: V
1
= first applied oltage
V
2
= second applied voltage
R
1
= resistance with V
1
applied
R
2
= resistance with V
2
applied
Since high resistance values are involved, it is imperative
that only the highest-quality components be used for the test
fixture. All insulators throughout should be made of a mate-
rial with very high resistance such as Teflon. Any residual
leakage current can be nulled out by suppressing the mea-
sured current.
3.7.3 Low-current measurements
Both the Models 6521 and 6522 are ideal for low-current
measurements because of their low offset currents. Also,
when the Model 6517 is programmed for the current scan
mode, the HI and LO terminals of all off channels are shorted
together, an important requirement when switching current
sources. (See paragraph 3.4.4 for information on program-
ming the Model 6517 for the current scan mode.)
Figure 3-8 demonstrates an application using low-current
switching, with the current sources connected to the channel
inputs as usual. In this example, channel 1 is closed so that
current from the source fl ws into the electrometer INPUT
jack. All other channels are open, but their ground relays re-
main closed so that the current paths are not broken.
Figure 3-7
High-resistance test system
Model 6517 Electrometer
HI
1
10
R
1
R
10
6521/22 Card
Resistors under test
Note: Electrometer must be configured
for internal voltage source connection
Triax Cable
WARNING:
NO INTERNAL OPERATOR SERVICABLE PARTS,SERVICE BY QUALIFIED PERSONNEL ONLY.
WARNING:
NO INTERNAL OPERATOR SERVICABLE PARTS,SERVICE BY QUALIFIED PERSONNEL ONLY.
CAUTION:
FOR CONTINUED PROTECTION AGAINST FIRE HAZARD,REPLACE FUSE WITH SAME TYPE AND RATING.
CAUTION:
FOR CONTINUED PROTECTION AGAINST FIRE HAZARD,REPLACE FUSE WITH SAME TYPE AND RATING.
INPUT
250V PEAK
!
LINE RATING
50-60HZ
50VA MAX
AC ONLY
LINE FUSE
SLOWBLOW
1/2A 90-125V
1/4A 180-250V
IEEE-488
(CHANGE IEEE ADDRESS
WITH FRONT PANEL MENU)
DIGITAL
I/O
TRIG LINK
115V
Voltage source HI