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Zero
Calibration
It is
common
practice
to
accept a small
offset in the
output of a
voltage
calibration standard,
providing
that
the
same
offset
is
present
at
all
output
values, including zero.
The
output
of
the
4708
is
fully floating,
so
its
output
may
be
referred
to
any
common
mode
voltage within
the
range
specified
on page
6.1.
In
particular,
its
zero
may
be
aligned
to
absolute
zero
in
Local Sense
by
calibration to
a
null
across
its
Hi and
Lo
(Sense)
terminals.
But
if
it
is
then gain-calibrated against
an
offset
standard without re-zeroing
to that
standard's
offset zero,
normal
mode
gain
errors will
result.
It is
therefore essential
that
each
voltage
and
current
range zero
is first
calibrated to
a
standard's zero
before using
that
standard
to calibrate the
range
gain.
If
the
4708
zero output
is
to
be
regarded
as absolute
Laboratory
Reference
Zero, then
AFTER
range
gain
calibration
its
range
zero output
may
be
recalibrated
to
a
null
across the
Hi and
Lo
(Sense)
terminals.
Equipment
Required
for
DC
Caiibration
DC
Voltage
-
AS
tandard
DC
Voltage source
of
suitable
accuracy:
Example:
Series
bank
of 10 standard
cells
and
Datron
4904
Standard
Cell
Buffer.
A
Precision Divider:
Example:
Datron
4902 High
Voltage
Divider
A
battery-operated
null
detector
with
variable
sensitivity,
able
to
withstand
1200V
across
its
input terminals:
Example:
Keithley Instruments
Model
155
DC
Current
-
A DC
Voltage
source, calibrated
to
suit-
able
accuracy
at
approximately
IV
and
lOOmV:
The
battery-operated
null
detector
used
for
DC
Voltage
A
set
of
calibrated current
shunts
of
suit-
able accuracy.
N.B.
To
allow
the
same
value
to
be
set
on
the
DC
Voltage source
for
each
range, the shunts
may
be
of
five
decade
values.
Then
the
same
Null
Detector
sensitivity
can be used
on
each
range.
CAUTION
When
choosing
a
set
of
current shunts ensure
that
their
power
dissipation ratings are
sufficient to
avoid
permanent
degradation
from
the
self-heat-
ing
effects
of
the current
being checked.
This
applies particularly
to the
1
Amp
shunL
alternatively,
a
DMM
of
sufficient
accu-
racy
may
be used
to
measure
the voltage
across the
set
of
calibrated current
shunts.
Example:
Datron 1281
using
"compute"
mode.
Resistance
-
a
set
of standard
resistors
covering
1
OO
to
lOOMO.
The
lOO
to
lOkO
should be
4-
wire
type.
an
accurate resistance bridge, or other
ratiometric
device
for
measuring
resis-
tance
to
the required accuracy.
a
Datron 1281 used
as
a
transfer-measure-
ment
device.
Interconnections
Interconnection
instructions
in this
section are necessarily
simple
and
basic,
and
are
mainly
intended
to
show
connections
to
the
4708.
It
is
recognized
that
they
may
need
to
be
adapted
to
meet an
individual user's requirements.
It is
assumed
that
users
will
possess
knowledge
of
the
operation
and
use
of
standards
equipment such
as
that
mentioned
above.
DC
Calibration
Sequence
The
sequence of
operations
for
DC
calibration
of a
4708
is
given
below:
Preparation
DC
Voltage
DC
Current
Resistance
Return
to
Use
If
only a
partial
recalibration
is
to
be
done,
step
1
of
the
DC
Voltage sequence should be
carried
out
immediately
after
the
preparation.
WARNING:
During Performance
checks
and
calibration
a
common
mode
voltage
equal
to
the
full
range
voltage
may
be
present
at the
Null Detector
input terminals.
On
±1000V
checks
this volt-
age
is
potentially
lethal,
so
EXTREME
CAU-
TION
must
be observed
when
making
adjust-
ments
to
the
null
detector
sensitivity.
8-3
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