Keithley 237

1.4
GUARD
AND
REMOTE
SENSING
Guard
The
purpose
of
guarding
is to eliminate the effects
of
leakage current (and capacitance)
that
exists between
output
high
and
output
low.
In
the
absence of a driven
guard, leakage could
be
enough
to adversely affect
the
performance
of
the
Source
Measure
Unit.
Effective
guarding
is achieved
by
surrounding
output
high
with
a driven guard. Since
the
voltage potentials of
output
high
and
the
driven
guard
are
the
same, virtually
no
leakage current can flow from
output
high to
output
low.
The Source Measure Unit
provides a driven
guard
for
OUTPUT
HI
and
SENSE HI. The driven
guard
is always
enabled
and
thus, always available
at
the
rear
panel
of
the
Source
Measure
Unit.
When
the
Source Measure Unit
is
properly connected to a test fixture (such as
the
8006),
guard
is extended to
that
test fixture. Thus,leakage
in
the
connecting triax cables is virtually eliminated. Inside the
test fixture, coaxial cables can
be
used
to extend
guard
all
the
way
to the OUT.
Guard
should always
be
used
when
sourcing
or
measur-
ing
low current
(<lJ.IA)
or
for high
speed
measurements
(SOM
cycle <10msec).
WARNING
GUARD
is
at
the
same
potential
as
OUTPUT
HI.
Thus
if
lethal
voltages are
present
at
OUTPUT
HI,
they
are also
present
at
GUARD.
Remote
Sense
The Source Measure
Unit
has local
or
remote sensing ca-
pability.
When
using remote sensing, sense lines are ex-
tended to
the
OUT.
When
sourcing
V,
remote sensing allows
the
voltage to
be
sensed (measured)
at
the load for better regulation.
If
the
remotely sensed voltage
at
the DUT is lower
than
the
programmed voltage level
(due
to test lead IR drop), the
actual
output
voltage will
be
increased until
the
sensed
voltage equals the
programmed
voltage. This ensures
that
the exact
programmed
voltage appears
at
the
DUT.
+
+
SECTION1
Getting
Started
When
sourcing I, remote sensing performs
the
voltage
measurement right
at
the DUT. The measured reading is
the
actual voltage across the DUT
and
does
not
include
the IR
drop
of
the test leads.
Remote sensing
should
be
used
when
sourcing
or
meas-
uring
lm.A
or
more.
WARNING
With
remote
sensing
enabled,
an
open
sense
lead
will
result
in
lethal
voltages
appearing
at
OUTPUT
HI
and
GUARD.
This
voltage
can cause
injury
or
death,
and
damage
exter-
nal
circuitry. Always
make
sure
that
the
sense
leads
are
properly
connected before
enabling
remote sense.
NEVER
change con-
nections
with
power
applied.
Be
sure
to
al-
ways
discharge
and/or
disconnect external
power
sources.
1.5
OPERATING BOUNDARIES
Depending
on
how
it
is
programmed
and
what
is con-
nected to
the
output
(passive
or
active load),
the
Source
Measure
Unit
can operate
in
any
of
the
four quadrants.
The source-measure capabilities of the Models 236, 237,
and
238
are
summarized
in
Figure 1-8. From these
graphs,
it
can
be
determined
that
if
the
Models 236, 237,
and
238 are sourcing
llOV,
they can measure (and limit)
up
to
lOOm.A.
Conversely,
if
they
are
sourcing
lOOm.A
they can measure (and limit)
up
to
llOV.
Figure 1-8B shows the extended capabilities
of
the
Model
237.
When
sourcing over llOV
(up
to
llOOV),
it
can meas-
ure
(and limit) lOrnA. Conversely,
when
sourcing
lOrnA
or
less,
it
can
measure
(and limit)
llOOV.
Figure 1-8C
shows the extended capabilities
of
the
Model238. When
sourcing over
lOOm.A
(up
to
lA),
it
can measure (and
limit) lSV. Conversely,
when
sourcing
lSV
or
less, it can
measure (and limit lA).
When
operating
in
the
first
or
third
quandrant, the
Source Measure
Unit
is operating as a source. That is, the
Source Measure
Unit
is delivering
power
to a load. When
operating
in
the
second
or
fourth quadrant,
the
Source
Measure
Unit
is operating as a sink. As a sink,
it
is dissi-
pating
power
rather
than
sourcing it.
An
external source
or
an
energy storage device,
such
as a capacitor
or
a bat-
tery, can force operation
in
the
sink region. For example,
if
a 12V
battery is connected to a Source Measure Unit
(Output
HI
to battery high)
that
is
programmed
for+
lOV,
1-9