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7
,_
Model
3484A
3-44.
LOW RESISTANCE
MEASUREMENTS.
When measuring low
values
of
resistance,
particularly on
the
I
00
D.
range,
internal
wiring resistance
and
test
lead
resistance
become
a significant
part
of
the measurement
display. The
internal
wiring resistance
may be
as
high as 50
milliohms
on the
100 n
range
(5
counts
in
the
least
significant
digit) with
a
direct
short across the input
terminals.
Before making a low
resistance measurement,
short the test
leads
together
and
note the
offset
shown
in
the
display.
This offset represents test
lead
and wiring
resistance
and
should be
subtracted from the
resistance
measurement. Do not
attempt
to remove
the offset by
adjusting the
front panel ZERO control.
This
will
result in
incorrect
measurements
on
all
ranges
and functions.
3-45.
RESISTANCE MEASUREMENT PROCEDURE.
a.
Turn instrument on
and
allow
to
warm up for
one
hour.
b.
If
resistance to
be measured is
within
a
circuit,
make sure power
to
the
circuit is
t
urned
off and
that
no
residual
voltages
are
present.
c.
Connect GUARD
terminal to
LOW.
d. Set FUNCTION
switch
to OHMS.
e.
Select
RANGE
manually or remotely
, or set to
AUTO.
f.
Adjust
SAMPLE
RATE
control
to
desired
sam-
pling
speed, or
apply
external
trigger.
Refer
to
Paragraph
3-7.
NOTE
When making
measurements
on the
1000 kn
or
10 Mn
ranges,
refer
to Para-
graph 3-39.
When
making measurements
on
the
100
n or
1000
n ranges,
refer
to
Paragraph 344.
g.
Select
FRONT
or
REAR
terminals and connect
resistance
to
be
measured
across
HIGH
and LOW
terminals and read measurement
on
front panel
display.
3-46. AC VOLTAGE
MEASUREMENTS (OPTION 043).
3-47. AC OR DC COUPLED MEASUREMENT.
The
V AC(AC) function
permits
true
rms voltage
measure-
ments of
frequencies
from
20
Hz
to
J
MHz.
The
V
AC(DC)
mode
extends
the
frequency
range
down
to
1
Hz,
and
permits true
rms
measurement of ac
plus de. The response
to
a
measurement
containing
both
ac and
de is equal to
V(DC)2+(ACrms)2.
In
addition,
measurement
of
the
ac
component of signals
down
to
I
Hz
may
be
made
by
adding
an
external
10
µF
blocking capacitor.
Section III
3-48. MAXIMUM
INPUT VOL TAG
ES.
3-49
. Maximum input
voltages
permissible
in the
VAC(DC)
function are
1500 V
peak
ac,
1500 Vdc, or a
total
peak
value
(ac
+de) of 1500
V.
3-50. In
the
V
AC(AC) fun
ction,
peak ac input limit
is
1500V
and total
peak
voltage limit
(ac+dc)
is
1500V.
The de voltage
input
limit
on the
JOO mV
range
is
IO V,
and
100 Von all
other
ranges.
The
ac
component
of inputs
having
a
de component
greater
than
the
limits
shown
may
be measured if
an
external
J
0
µF
blocking
capacitor is
connected
in
series
with
the
input HIGH terminal.
The
capacitor
must
have a
voltage
rating
greater
than
the
de
com ponent of the
input
voltage.
Leakage
current
through
the
external
capacitor must
not
exceed
1
µ.A.
3-51.
RESPONSE TIME.
Response
time
in
the
V AC(AC)
mode is
1 second
to
within
5 counts
of
final
reading.
In
the
V AC(DC)
mode,
response
time is
15 seconds
to
within
5 counts
of
final
reading.
FILTER switch
setting
has
no effect
on
either
response
time
or filtering.
3-52. MEASURING SPEED.
The automatic
sampling rate
is
variable
from
one
per
second
to
25 per
second
with
the fro nt
panel
control.
When
the
control
is
in
the
HOLD
position
or
the Interface
Hold
{Inhibit) Line is
grounded,
the
instrument may
be triggered
manually
by
means of
the
TRIG pushbutton
, or
remotely
up
to
1000
times
per second. The
slower
response time
of
the ac
converter
should
be
taken
into
consideration when
determining the
remote triggering rate,
which
is
discussed
in
Paragraph
3-15.
3-53.
AUTORANGING
TIME.
Autoranging
time
(per
range
change)
is
I
second
for
the
V AC(AC)
mode
and
3 seconds for
the
V
AC(DC)
mode.
FILTER
switch setting does
not
affect
autoranging time
in
either ac
fun
ction. When autoranging occurs,
the
Print
Command
Output remains HIGH until
after
the
correct
range is reached
and a measurement
is
taken on that
range.
This
prevents
a
recorder
from printing
erroneous readings
during
autoranging.
3-54.
AC VOLTAGE MEASUREMENT PROCEDURE.
a.
Turn
instrument on and
allow to warm up for
one
hour.
b.
Set
FUNCTION
switch
to
V AC(AC) or V
AC(DC).
See
Paragraph
347.
c.
Select RANGE manually
or
remotely,
or
set
switch
to
AUTO.
d.
Adjust
SAMPLE RATE control
to
desired
sam·
3-7
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