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HP 3468A - Optimizing Reading Rates

HP 3468A
114 pages
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There
are
several
reasons
why
you
would
want
to
optimize
the
rate
at
which
readings
are
taken
by
the
3468A.
Perhaps
you
are
using
a
scanner
to
measure
a
large
number
of
points
where
a
faster
reading
rate
would
mean
a
better
picture
of
what
is
happening
at
a
single
point
in
time.
Or
maybe
you
need
to
read
fast
so
that
you
don’t
waste
valuable
computer
time
waiting
for
a
measurement
result.
Whatever
your
reason
for
optimizing
reading
rates,
the
3468A
can
solve
many
of
these
application
problems.
Your
maximum
reading
rate
with
the
3468A
is
in-
fluenced
by
several
factors.
These
include
the
signal
environment
(line
related
and
broadband
noise,
thermals,
etc.),
the
desired
accuracy,
and
convenience
features
such
as
autorange
or
autozero.
The
speed
and
timing
of
the
A/D
process
is
depen-
dent
upon
a
number
of
factors.
The
number
of
digits
of
resolution
selected,
whether
or
not
the
autozero
feature
is
enabled,
and
the
selected function
deter-
mine
how
long
it
takes
for
the
A/D
to
make
a
conversion.
nvironment
The
signal
that
you
are
trying
to
measure
is
subject
to
line
related
and
broadband
noise
which
can
interfere
with
your
measurement.
The
3468A
works
to
reduce
or
reject
this
kind
of
noise
by
using
a
form
of
Analog
to
Digital
(A/D)
conversion
call-
ed
integration.
Integration
is
a
process
where
line
related
noise
is
averaged
to
zero
over
the
period
of
an
integral
number
of
power
line
cycles
(PLC’s)
during
an
A/D
conversion.
The
measure
of
the
ability
of
the
multimeter
to
measure
de
voltages
in
the
presence
of
ac
voltages
(at
power
line
frequencies)
is
called
Normal
Mode
Re-
jection
(NMR).
The
NMR
of
the
3468A
is
dependent
upon
the
number
of
digits
displayed.
An
important
part
of
this
process
is
to
make
certain
that
the
50/60
Hz
line
switch
(S1
on
the
rear
panel)
is
set
properly:
up
for
50Hz
line
frequency
and
down
for
60
Hz.
Integt
Changing
the
number
of
digits
of
display
does
more
than
change
the
resolution
of
the
reading.
It
actually
changes
the
“INTEGRATION
TIME”
which
determines
the
reading
rate.
The
integration
time
is
not
the
same
as
the
time
for
one
measure-
ment,
the
integration
time
is
the
time
period,
in
PLC’s,
during
which
the
voltmeter
samples
the
input
voltage.
At
4
1/2
digits
of
display,
the
time
required
for
one
in-
tegration
period
is
one
PLC:
16
2/3
mS
at
60Hz
line
frequency
or
20mS
at
50Hz.
At
3
1/2
digits
of
display,
the
integration
time
is .1
PLC.
Normal
Mode
Rejection
(NMR)
is
a
measure
of
the
ability
of
the
voltmeter
to
accurately
measure
dc
voltages
in
the
presence
of
ac
voltages
at
power
line
frequencies.
The
3468A
has
much
better
NMR
at
the
4
1/2
digits
of
display
(59
db)
than
at
3
1/2
digits
(Odb)
because
of
the
integration
times.
At
5
1/2
digits
of
display,
the
3468A
takes
ten
(10)
readings,
spaced
a
fraction
of
a
line
cycle
apart,
in
the
4
1/2
digit
mode
and
averages
them
together.
This
provides
the
greatest
amount
of
noise
rejection
(80db).
A?

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