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4.9 STATIC CHARGE DETECTION
Electrostatic
charge is a deficiency
or excess of electrons
on an
ungrounded
surface.
Such charges are usually
generated
on
poor
conductors
of electricity
such
as plastics,
synthetic
fibers,
and
paper
during
handling
or processing
of these
materials.
Once these charges accumulate,
they do not dis-
sipate
readily
because
of the excellent
insulating
character-
istics of the materials
involved.
Static charge build-up
can be a problem
with integrated
cir-
cuits, especially
with those of the CMOS variety.
While these
devices,
which operate
at high impedance
levels, often have
static protection
built in, it is best to properly
protect
them
during transit or storage. For that reason, such KS are usually
shipped
and stored in anti-static
tubes.
A primary
consideration,
then, is the degree of static protec-
tion afforded
by the anti-static
tube. A comparison
among
various
tubes can be set up to test the variations
in charge
build-up
as a particular
IC slides the length of the tube. The
charge value will, of course, be measured
by the Model 617
being operated
in the coulombs
function.
To perform
this test, a test fixture called a Faraday
cup will be
necessary.
Such a fixture can be easily constructed
from two
cans,
as shown
in Figure
4-11.
For example,
the outer can
could be the ubiquitous
one-gallon
paint can, while the inner
cylinder
could be one of slightly smaller diameter,
such as a
quart
paint
can. The two cans must be insulated
from one
another.
Although
the type
of insulator
is not
all that
critical,
ceramic
or Teflon insulators
can be used.
INSULATORS
iTEFLON OR CERAMIC)
Figure 4-11. Faraday Cup Construction
For convenience,
a BNC connector
could be mounted
on the
outside
can. The outer,
or shield connection
will, of course,
be connected
to the outer
can, while
the inner
conductor
should be connected
to the inner can.
To perform
the test, connect
the Model
617 to the Faraday
cup using a suitable
shielded cable, such as Model 4801 BNC
cable. A Model 6147 triax-to-BNC
adapter
will be required
to
make the connection.
With the instrument
in the coulombs
mode,
place a typical IC in the tube to be tested; allow it to
slide the full length of the tube and fall into the Faraday
cup.
The amount
of charge built up during
the test will then be
registered
on the Model
617.
The test can be repeated
with other
tubes,
as required.
In
order for the test to be valid,
all tubes should
be the same
length,
and the same IC should
be used in every case. The
tube that generates
the smallest
static charge as seen on the
electrometer
is the one with the best anti-static
characteristics.
The amount
of charge
seen during
this test will depend
on
many
factors,
including
the
type
of tube
material,
tube
length,
the IC used, as well as
the
relative
humidity.
Typical
values might be in the 0.5-1°C
range for a good anti-static
tube, while one without
anti-static
protection
might generate
10 times that amount.
4.10 USING THE MODEL 617 WITH
EXTERNAL VOLTAGE
SOURCES
The internal
voltage source of the Model 617 should be more
than adequate
for most measuring
situations.
However,
there
may be a few applications
where a voltage
higher than
the
nominal
+lOOV value
is required.
For example,
it may be
desirable
to increase the measurement
range of the V/I ohms
mode. In another
instance,
voltage coefficient
studies at high
voltages
may
required.
These
functions
can be perfamed
with
the
Model 617 if an external
high voltage source is used.
Accuracy
of the V/I ohms mode will depend
largely
on the
relative
current
seen by the instrument.
For best accuracy
in
this mode, it is best to choose a range that will result in a cur-
rent that is equal to a large percentage
of the full range value.
On the 2013TQ V/I range, for example,
a full range resistance
measurement
will result
in a current
of 0.5pA.
assuming
a
voltage
of 1OOV is being used. For resistances
above ZOOTQ,
the current seen by the instrument
will be less than 0.5pA. For
very high resistance
values
(above
2pR). the current
will be
very small indeed,
and accuracy
will be reduced.
Figure 4-12 shows a test set-up
using the Model
617 along
with
an external
supply
to make
V/I
resistance
measure-
ments. The basic set-up is much like that used when making
V/I measurements
with
the Model
617 voltage
source:
the
resistance
under
test is connected
in series with
the elec-
trometer
input lead. The voltage supplied
by the external sup-
ply
forces
a current,
which
is read
by the electrometer,
through
the resistor.
The current
and voltage values are then
used to calculate
the resistance.
4-12
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