METER Indicated the
magnitude and polarity of
the
measured
voltage on the TVM
mode. On the
differential mode/ the meter+or-deflection indicates
that the
measured
voltage is above
or
below the readout dial indication.
Figure
2-2.
THE MODEL 893AR CONTROLS, TERMINALS, AND
INDICATORS (Sheet
2 of
2)
b.
Touch the HI INPUT terminal and the chassis of the
instrument with your hand, oteerving that the
meter
pointer deflects to the right or left.
c. Place the MODE switch to the -DC position and re-
peat step
b.
The meter pointer should deflect in the
opposite direction
by
approximately the
same a-
mount observed in step b. Place the MODE switch
to the +DC
position upon completion
of
this step.
d.
Remove the shorting strap from between the LO and
groxmd
INPUT terminals
and
connect the
strap be-
tween the
HI
and LO
INPUT terminals.
e.
Perform the Readout dials versus NULL SENS
checks of
Figure
2-3,
observing that the meter
pointer indicates
-1
for each particular check.
READOUT DIALS
NULL
SENS
OOOX
.001
. 00100 .001
.01000 .01
.
10000
.1
Figure
2-3. READOUT
DIALS VERSUS
NULL
SENS CHECKS
f.
Place
the Readout
dials and NULL SENS
switches
to their respective .
10000
and
.
1
positions,
observ-
ing that the
meter
pointer
indicates
-1.
g.
Retaining the
control
positions of step f,
place the
RANGE
switch to
each available
position,
observing
that the meter
pointer
indicates
-1
in all
positions
of the RANGE
switch.
h. Place
the Model 893
controls
to the following
posi-
tions:
POWER
ON/LINE
OPR
RANGE
1
NULL SENS
.001
MODE
AC
Readout dials
000. 00
i.
Touch the
HI
INPUT terminal of
the Model 893,
observing
that a deflection
is obtained on
the meter.
2-14.
If
the results of the
C^erational Test agree
with
information given,
it can
be
assumed
that the instru-
ment is
operating properly
and measurements of ac
and
dc
voltages can now
be performed.
2-15.
OPERATING NOTES
2-16.
GROUND
LOOP CURRENTS
2-17.
A
potential
difference often exists
between dif-
ferent points of
power system grounds.
Consequently,
2-4
current may flow from one power system ground through
the voltmeter and the voltage source being measured to
another power system ground during the measurement
process. These ground loop currents should be avoided
as
they generate voltages that degrade measurement ac-
curacy. To prevent these
ground
loop currents, when
the system being measured is groimded, remove
the
shorting strap between the LO INPUT terminal and the
ground terminal of the Model 893 or, if
your
instrument
is
equipped
with the
-01
Option,
operate the
instrument
from
battery
power
with
the line cord
disconnected from
line power.
2-18.
COMMON MODE VOLTAGES
2-19.
Great care has been
taken in the design
and
con-
struction
of
this
instrument to isolate the circuitry from
chassis ground to reduce common mode measurement
errors caused partly
by
leakage currents. Accurate
dc measurements can be made in the presence
of
com-
mon mode voltages up to 500 volts
dc
or
ac
voltages up
to 100 volts peak-to-peak at 50 to 500 Hz. If the com-
mon mode voltage is greater than 50
volts,
the meas-
urement should
be made several
minutes after the
equipment connections for best accuracy due to the time
required to charge the stray capacitance through the
extremely high
leakage
resistance to ground.
2-20. USE OF SHORTING LINK
2-21.
The INPUT
ground terminal is directly connected
to
the chassis of the instrument
to
provide a
convenient
connection to
earth ground whenever the line cord
is
connected
to
line
power.
Whenever measurements of
voltages
having
a
reference other than ground,
but
not ex-
ceeding 500 volts, are performed,
the shorting link be-
tween the LO INPUT and ground
terminal must be re-
moved or damage
to the measurement source,
the Model
893,
or both may
occur.
2-22.
DC
DIFFERENTIAL
OFF-NULL INPUT
RE-
SISTANCE
2-23.
Input
resistance
of
the Model 893 is infinite at
null during dc differential measurements
because no
current flows between
the measured source and the in-
ternal reference
supply
of
the instrument. However,
a small current does flow
between the measured source
and the internal reference supply
whenever the instru-
ment is not exactly
nulled. For example, when the
meter deflection is
10%
of full-scale on
the 0. 001 volt
NULL SENS raxige, the
input resistance of the instru-
ment
is
10^^
ohms per
volt
of
the measured Input volt-
age or
10^2
ohms for an inpiit measured voltage of 10
volts dc. A graph of
these
apparent
off-null input re-
sistances is contained in Figure
2-4.
893A
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