OPERATION
MODEL
263
(GUARD ENABLE)
MODEL 263
(GUARD ENABLE)
INNERSHIELD INNERSHIELD
r------ r------
I I
’ PR ’ PR
; ou ; ou
I I
I I
I I
I I
I I
I I
-----------------J
-----------------J
“G=“OUT “G=“OUT
Figure 3-9. Guarded Circuit Figure 3-9. Guarded Circuit
3.7.1 Guarded Ohms
WARNING
Hazardous voltage may be present on the in-
ner shield of the OUTPUT connector when
sourcing guarded ohms. A safety shield (outer
shield of the supplied triax cable) connected to
earth ground should be used.
Guarding is recommended for resistances zlOOM0. The
unguarded and guarded output configuration for ohms
is shown in Figure 3-10. With guard enabled, output low
is physically disconnected from the inner shield of the
OUTPUT connector on the rear panel of the Model 263.
This allows guard drive to be connected to the inner
shield. In OHMS, guard drive (the inner shield of the t&x
connector and cable) must be provided by the external
DUT.
Most electrometers have an output connector that pro-
vides a guard drive. Some electrometers (such as the
Keithley Model 617) have the capability to reconfigure its
input (through means of a switch) and internally connect
the guard drive to the inner shield. With this capability,
connecting the Model 263 to the electrometer is a simple
matter as shown in Figure 3-11. For electrometers whose
input cannot be reconfigured, an input adapter will be re-
quired. The objective of the adapter is to connect the guard
source of the electrometer to the inner shield (guard) of
the Model 263. Two such input adapters are available from
Keithley. The Model 6167 is used with Keithley Model 614
Electrometer, while the Model 6191 is used with the
Keithley Model 619 Electrometer. Figure 3-12 shows how
the Model 263 is connected to an electrometer (Model 619)
using an input adapter (Model 6191). Note that in Figures
3-11 and 3-12 source low is routed to the electrometer us-
ing a separate banana plug cable. This cable eliminates the
need to route source low through earth ground connec-
tions that may have higher resistance.
In some electrometers there is a resistor (typically 100Cl
or lk0) connected between common and low (see Figure
3-11). In guarded ohms this resistor is in series with the
output resistor of the Model 263. In this situation, never
use guard to source c100MO to the Model 617 and
110MIl to the Model 614. In the unguarded configura-
tion, source low is routed directly to meter low.
Table 3-2 summarizes the techniques to source guarded
ohms to Keithley electrometers.
3-11
To Next Page
Loading...
Need help?
General
