Datron 4708 User Handbook Manual page 114

General Procedural Information
Appendix
3
to:
4708
User's
Handbook
Section
7
Notes
on
the
use
of
the
Null
Detector
(DC
Voltage Ranges)
The
null detector
is
normally connected
in series
with
the
4708 Hi
lead.
A
high-impedance-input device should be
chosen
to
reduce
off-null
currents
due
to differences in the
outputs
of the
DC
voltage source and
the
4708.
A
battery-
operated instrument
is
preferred
to
ensure adequate
isolation.
Some
null
detectors
possess high input
impedance
only
when
their
readings are on-scale,
so care should be taken
to
ensure
that
drain currents
from
the
DC
Voltage source
do
not
become
excessive. This applies particularly
if
the
DC
source
is
a
standard
cell
or a
bank
of
cells.
Six points are important:
1
.
The
null detector
should
be connected
to
the
4708
(or
4708
load
resistor)
only
when
the
4708
OUTPUT OFF
LED
is lit
(With Output
OFF,
the
I+,
I-,
Hi and
Lo
terminals are
at
high impedance).
2
.
Always
set
the null detector to
its
lowest
sensitivity
before connecting
up,
and
increase
sensitivity
only
when
the
voltages output by
the
DC
Voltage source
and
the
4708
are close in value.
3
.
Do
not
change
polarity
of
the
4708
or
DC
Voltage
source without
first
switching
the
4708
OUTPUT
OFF.
Care must be
taken to
ensure
that
the
correct-
polarity
ON
key
is
pressed, to
avoid excessive
voltages
being connected
across the null detector,
particularly
when
checking
the
4708
directly
against a
standard
cell.
4
.
Most
Null Detectors are
equipped
with a
'Self-zero'
or
'Zero-check'
facility.
For
maximum
accuracy, the
Null
Detector range zero should
be checked
before
each
nulling operation
is
performed.
However,
when
verifying the
4708 Voltage ranges, the zero
offset
of
the
DC
Standard voltage source
is
nullified
by
adjustment of
the
Null Detector zero
control.
This
setting
should not
be
altered until the
corresponding
range gain has been
verified.
5.
WARNING
During Performance checks
and
calibration
a
common
mode
voltage
equal
to the
full
range
voltage
is
present
at
the Null Detector input
terminals.
On
lOOOV checks
this
voltage
is
potentially lethal,
so
EXTREME CAUTION
must
be observed
when making
adjustments
to
the null
detector
sensitivity.
6.
CAUTION
The
Null Detector used
must be
able to
withstand
voltages
up
to
1200V between
its
input terminals.
Such
voltages will
be present
during
the
time
that the
4708
is
ramping from
zero
to
lOOOV
Full
Range
after
setting
OUTPUT
ON.
Inadvertent disconnection of
the
Precision Divider terminals
can
transfer
full
output
across the
Null
detector.
7-A3-1