Fluke 8502A Instruction Manual page 15

Although not
visible
in
Hi
Res, the
exponent
remains
in
effect; care
should be
exercised
when
interpreting readings.
To
avoid confusion,
remember
that an
exponent
in the
exponent
display
will
always have
its
own
polarity
sign; a
high resolution
digit will
not.
2'25.
Input Terminals
2-26.
TERMINAL
INTERCONNECTIONS
2-30. Correct use of
the
8502A's
guard terminal
will
both protect the instrument and provide
more
accurate
readings.
Since the
LO
to
GUARD
maximum
voltage rating
is
127V,
the guard terminal should always be connected
either
to the
INPUT
LO
terminal
or to a point in the
source
circuit
to be measured. For
accurate readings,
connect
the
guard to
minimize
common mode
currents
flowing through
any
resistance
which
helps determine the voltage being mea-
sured.
See Figure
2-3, for
su^ested guard
connections.
2-27. For dc voltage measurements, both
INPUT
HI
terminals
may
be
left
strapped
together,
The
INPUT LO
terminals
may
likewise
remain
strapped.
For most mea-
surements,
it
is
sufficient to strap
GUARD
to
INPUT
LO.
Refer
to
Figure
2-2, for
recommended
input
terminal
strapping.
CAUTfOIM
The
guard
terminal
should
not
be
left dis-
connected. Instrument
damage can occur
if
common
mode
voltage
exceeds the
LO
to
GUARD
maximum
voltage rating (127V).
Figure 2-2.
Front Panel Terminal Interconnections
2-28.
GUARDING
2-31. Generally, guarding should be used
in
the following
situations:
I
When
long
signal leads are used.
2-29.
Common
mode
voltages, resulting
from
currents
and
voltage
drops between two
points otherwise
electrically
common, may
produce significant errors. Proper use of
a
floating,
guarded voltmeter
will
minimize
these errors
and
allow representative reading of
normal
mode
voltages (actual
source
voltage
and
noise).
2.
When
signal source
impedance
is
high.
3.
When
making measurements
near
high-level
radi-
ated
no^e,
particularly at the
power
line fre-
quency.
4.
When making
floating
measurements.