Keithley 237 - Page 80

SECTION2
Operation
2.6.3 Model 7072
Semiconductor Matrix
Card
The Model7072 is
an
8
x
12 matrix
card
that
is controlled
by
the Model707 Switching Matrix. Figure 2-24 shows a
two
Source Measure
Unit
test system
using
two
Model
7072
matrix cards.
If
more
OUT
pins are needed,
the
sys-
tem
is easily
expanded
by
adding
more Model7072 ma-
trix cards. Each additional matrix
card
will
add
12 col-
umns
to the test system.
Rows A
and
B
of
the matrix cards, which
have
low-cur-
rent offset (<1pA), can
be
used
for
low
current sourcing
or
measurement applications. Rows
C, D,
E
and
Fare
general purpose
paths
for situations
that
do
not
require
low-current performance. Rows
G
and
H
provide
single-
pole switching.
One
of
these rows could
be
used
as a com-
mon
low, while
the
second
one
could
be
used
to source a
low impedance (not sensitive) bias voltage to
the
OUT.
2-28
+
+
At
the test fixture, the
drain
and
source leads
of
the PETs
are connected
in
a 4-wire sensing configuration. This con-
nection scheme allows
Source
Measure Unit
#1
to
use
re-
mote sensing to accurately
apply
Vos
to
the
PETs.
Source
Measure
Unit #2 uses local sense
and
is
used
to
supply
the
bias to
the
gates of
the
PETs.
Since
the
gates are low
current, remote sensing is
not
necessary.
Source Measure
Unit
and
test fixture connections to the
matrix card are accomplished
using
Model7078-
TRX
ca-
bles. These are 3-slot triax cables.
On
each Source Meas-
ure
Unit,
the
banana
jack is
used
to access OUTPUT LO.
By
using
the adapters
shown
in
Figure 2-21,
the
OUTPUT
LO
banana
jack can
be
mated
to a 3-slot triax cable
(7078-
TRX).
This allows
OUTPUT LO
to
be
applied to a
signal
pathway
and
independently switched. The
guard
pathways
of
the
matrix cards are
used
exclusively to ex-
tend
the
driven
guards
of
the
Source
Measure
Units
to the
OUT
to eliminate
the
effects
of
leakage current.