Fluke 8505A Instruction Manual page 27

8505A
2-46=
Measurement Terminals
and
Controls
2-47.
GUARDING
2-48. General
2-49.
Guarding
may
be used to
reduce
noise
and
improve
accuracy.
Common
mode
voltages, resulting
from
currents
and
voltage
drops between two
points
otherwise electrically
common, may
cause
significant
errors.
Proper use of a
floating,
guarded
multimeter
minimizes
these
errors.
2-50.
Generally,
guarding should be
employed where
long
signal leads
are used,
when
signal
source
impedance
is
high,
when making
measurements near
high-level
radiated noise (particularly at the
power
line
frequency),
or
when making
floating
measurements.
NOTE
Errors due
to
thermal emfs should be
considered
when making low
level,
high
resolution
measurements.
Thermal emfs
(voltages
produced by temperature
differences
between contacts
of two
dissimilar
metals or by
temperature gradients along
a
length of
material)
may
cause
differences of
several microvolts.
The
use of
low
emf,
shielded cables with
copper spade
lugs
is
recommended
to
minimize thermal
emf
errors.
2-51.
Guard
Selector
2-52.
Correct use of the
multimeter
GUARD
terminal
both
protects the
instrument
and provides
more
accurate
readings.
The
EXT
GD
IN
selector,
when
disengaged
(out),
enables the internal
guard
connection.
In
many
cases
accurate
measurements
may
be
made
with
the
selector in this
position
(shown
as
A
in
Figure
2-4).
Here,
the difference
in
potential
between multimeter ground
and
device
ground
is
very
small,
or the
measurement
is
not
critical.
When
pushed
in,
the selector disables the
internal
guard connection
and
allows for
external guard
connections
as
shown
in
B and
C
of Figure
2-4.
The
connection
shown
in
B
is
better
than that
in
A,
since
some
common mode
current
(ICM)
is
shunted
away
from
the
source
resistance.
The
connection
shown
in
C
is
necessary
when
the source
may
not be capable of driving
the
guard.
The
buffer amplifier
shown
in
C
prevents
this
source
loading. Practical considerations usually dictate
which of
the three
connections
is
used.
2-53.
Guard
Terminal
2-54.
Recommended
guard terminal connections are
illustrated in
Figure
2-4.
The
potential
between
GUARD
and chassis
ground must not exceed
500V.
The
potential
between
SENSE LOand
GUARD
must
not exceed 127V.
2-55.
VOLTS INPUT/OHMS SENSE
(HI
and LO)
TERMINALS
2-56.
The
VOLTS INPUT/OHMS SENSE
terminals
are
used
when making voltage (dc
and/or
ac)
and
resistance
measurements. Connections
are
shown
in
Figure 2-5.
The
input
on
the
HI
terminal with respect
to
the
LO
terminal must
not
exceed 1000V.
The
LO
to
GUARD
potential
must
not exceed I27V.
These
terminals are internally
shorted
to the
AMPS
INPUT/OHMS SOURCE
terminals
(HI
to
HI,
LO
to
LO) when
the
Ohms
Selector (4T
OHMS
IN)
is
in
the
disengaged (2-wire) position.
NOTE
In
OHMS
measurements,
the voltage
at
the
SENSE
IN
PVT
terminals
is
sampled
before
the
Ohms
Converter (Option 02)
is
connected.
No
connection
is
made
if
excessive voltage
(±I0V
dc)
is
present.
2-57.
AMPS INPUT/OHMS
SOURCE
(HI
and LO)
TERMINALS
2-58.
The
AMPS INPUT/OHMS SOURCE
terminals
are used
when making
current
(A
DC
or
A
AC)
or four-
wire resistance
(OHMS)
measurements.
The
potential
between
SOURCE
HI and
SOURCE
LO
must
not
2-19