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Operation
Any
differential isolation
capacitance
affects
EXZ
mea-
surement
settling
time
as
well
as
AC
measurement
ac-
curacy.
Thus,
it
is
often
important
that
such
capacitance
be kept
as
low
as possible.
Although
the distributed ca-
pacitance
of
the
matrix card
is
generally fixed
by
de-
sign,
there
is
one
area
where you do have
control
over
the
capacitance
in
your
system;
the
connecting
cables.
To minimize
capacitance,
keep
all
cables as short as
possible.
4.5.2
Magnetic
fields
When
a conductor
cuts
through magnetic
lines
of
force,
a
very
small current
is
generated. This
phenomenon
will
frequently
cause
unwanted
signals to
occur
in the
test
leads
of
a switching
matrix system.
If
the
conduc-
tor
has
sufficient
length,
even
weak
magnetic
fields like
those
of the earth
can
create sufficient
signals
to affect
low-level
measurements.
Two
ways
to
reduce
these
effects are:
(1)
reduce
the
lengths
of the
test
leads,
and
(2)
miniinize the
exposed
circuit area.
In
extreme
cases,
magnetic
shielding
may
be
required. Special
metal with high permeability
at
low
flux
densities
(such as
mu
metal)
is
effective at re-
ducing
these
effects.
Even
when
the
conductor
is
stationary,
magnetically-
induced
signals
may
still
be
a problem.
Fields
can
be
produced by
various
signals
such
as the
AC
power
line
voltage.
Large inductors
such
as
power
transformers
can generate
substantial
magnetic
fields,
so
care
must
be taken
to
keep
the
switching
and measuring
circuits
a
good
distance
away
from
these potential noise
sources.
At high
current
levels,
even
a
single
conductor can gen-
erate significant
fields.
These
effects
can
be
miniinized
by
using
twisted
pairs,
which
will
cancel
out
most
of
the resulting
fields.
4.5.3
Radio frequency
interference
RFI (Radio Frequency
Interference)
is
a
general
term
used
to describe
electromagnetic interference
over a
wide
range
of
frequencies across the
spectrum.
Such
RFI can be
particularly
troublesome
at
low
signal lev-
els,
but
is
can
also
affect
measurements
at
high
levels
if
the
problem
is
of
sufficient severity.
RFI
can
be
caused
by
steady-state sources
such
as radio
or
TV
signals,
or
some
t)q?es
of electronic
equipment
(microprocessors,
high speed
digital
circuits,
etc.),
or
it
can
result
from impulse
sources, as in the case of
ardng
in
high-voltage environments. In
either case,
the
effect
on
the
measurement
can
be
considerable
if
enough
of
the
unwanted
signal
is
present.
RFI can be
minimized
in several
ways.
The most
obvi-
ous
method
is
to
keep
the
equipment and
signal leads
as
far
away
from
the
RFI source
as possible.
Shielding
the
matrix switching
card, signal leads, sources,
and
measuring
instruments
will
often
reduce
RFI
to
an
ac-
ceptable
level.
In
extreme
cases,
a
specially-construct-
ed
screen
room
may
be
required
to
sufficiently
attenuate the
troublesome
signal.
Many
instruments
incorporate
internal
filtering
that
may
help
to
reduce RFI
effects
in
some
situations.
In
some
cases,
additional external
filtering
may
also
be
re-
quired.
Keep
in
iriind,
however,
that
filtering
may
have
detrimental
effects
on
the desired
signal.
4.5.4
Ground
loops
When
two
or
more
instruments
are
connected
together,
care
must
be taken
to
avoid
unwanted
signals
caused
ground
loops.
Ground
loops usually occur
when
sensitive
instrumentation
is
connected
to
other
instru-
mentation with
more
than
one
signal return
path such
as
power
line
ground.
As shown
in
Figure
4-12,
the
re-
sulting
ground
loop
causes current
to
flow
through
the
instrument
LO
signal leads
and
then
back through
power
line
ground. This
circulating current
develops
a
small
but imdesirabie
voltage
between
the
LO
termi-
nals of the
two
instruments. This voltage
will
be
added
to
the
source
voltage, affecting the
accuracy of
the
mea-
surement.
4-14
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