Keithley 237

SECTION2
Operation
2.30 GUARDING
When sourcing
or
measuring low current, leakage
cur-
rent can
be
significant enough to adversely affect per-
formance. Guarding uses a conductor
at
essentially the
same potential seen
at
OUTPUT HI. The driven guard,
which is used to
surround
OUTPUT
HI
and
SENSE HI,
drastically reduces leakage current
and
the
effects of ca-
ble capacitance.
NOTE
Figure 2-78
and
Figure
2-79
are
used
to help
explain the principles
of
guarding. Both illus-
Figure
2-78.
Unguarded
Coaxial
Cable
+
trations
show
LO
being connected to chassis
ground.
On
the Source Measure
Unit, OUT-
PUT
LO
can
be
connected to chassis
ground
by installing the
ground
link
on
the rear panel.
Even
with
the
ground
link
removed, the resis-
tance between OUTPUT
LO
and
chassis
ground
(lOGO) is relatively low
when
com-
pared
to the insulation resistance of the cable
(lOOGO).
Thus, for the following discussion
on
low level leakage currents, the outer shield
of the cables (chassis ground) can
be
consid-
ered to
be
OUTPUT
LO.
-L,-
HI
100pF
LO
OUTER
SHIELD
Figure
2-79.
Guarded
Triax
Cable
2-112
+