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Section
V
the
3490A
accuracy.
In this case,
a
correction
factor
chart
should
be
used
to
adjust the
calibrator
output.
Such a
chart
for the
-hp-
'745A1746A
may
be derived
during
routine
periodic
calibration
of the instrument,
and
can
be used
to
set
the
745A controls
for
a
precise
output. For example,
if the
745A output
is known to
be
0.04%
high at I V, 20H2, the
745A
can
be
set
for a
precise
1 V output. by setting
the
745A voltage set
controls'to
1.000000
V and
adjusting the error
measure-
ment control
to
+0.04%.
The corrected
voltage
will
then
be
sufficiently
accurate
to test the
3490A
at
I V,
20H2.
To
determine the
length
of time that the
correction factor chart
will
be valid,
refer to the
latest
745A1746A Operating
and
Service Manuals.
t.
5-13.
To verify the
ac voltage
accuracy
of the
34904
at
frequencies
above I l0
kHz, the
ac to
dc transfer
measurement technique
may
be employed.
Using
the
test setup
shown
in Figure
5-1, apply
an
accurate dc
voltage to the
thermal
converter
and
adjust the
dc
differential voltmeter
for
a null indication.
Remove the
dc input
from
the thermal
converter
and
apply
the
ac
signal to
both the
thermal
converter
and
the
349OA
input
simultaneously.
Adjust the
test
oscillator
output
level
to
return the
differential
voltmeter
to
a null
indication.
This
results in
an rms value
of the
ac signal
equal to the
rms value
of the
dc standard
output.
5-14.
Table
5-3
lists
the voltage
and frequency
points
which
should
be checked
to
verify
the
ac voltage
accuracy
of
the
349OA, together
with the test
equip-
ment needed
for
each check.
If
the
equipment
required
for
some
checks
is not
available,
a
good
indication
of
performance
can
still
be obtained
by checking the points
for
which suitable
equipment
is
available. r-..
Model3490A
Connect Guard to Input Low or
damage
to
the instrument
may result.
5.15.
OHMMETER
ACCURACY TESTS.
5-l
6.
Preferred
Method.
5-17. A
resistance
decade with settings from 100Oto
10
MO is
required for this test.
A
correction
factor chart
for the
decade
is
necessary
to
achieve the
accuracy
required to check
the
performance
of
the
34904
'Ohmmeter
function.
Select
the
3490A ranges
and
resistance
decade settings shown
in Table
54. The
algebraic sum
of
the
34904
reading and the resistance
decade error indicated
by
the correction
factor chart
should
be within
the limits
given
for each
setting
listed.
5-l
8.
Alternate
Method.
5-19. If
a suitable resistance
decade with
an appropriate
correction
factor
chart is not
available, resistors
within
the
values
shown
in
Table
5-5, whose value is
known to
within the tolerances given,
may be used to
check
performance
of the
3490A ohmmeter
function. The
3490A
display should
be within the number
of counts
shown
from the
value of the resistor used.
5.20. DC
COMMON-MODE REJECTION
TEST.
5-21. Effective
common-mode
rejection is the ratio of
the
peak
common-mode
voltage
to the
resultant error in
PERFORMANCE CHECKS
t-,,,-----l
(O
)@u
o
@\:d
o-o
oo
MULTIMETER
hp 34904
o@oo@
DC
OIFFERENTIAL
VOLTMETER
/ RATIOMETER
hp 542O8
OU
T
PUT
CAELE
Figure
5-1. AC
Voltmeter
High
Frequency
Accuracy Test.