HP 3468A - Page 21

Do This |
press: This puts the 3468A into the 3 1/2
digit display mode. This mode has the
[A fastest reading rate but the lowest
resolution and little noise rejection.
The integration time in this mode is .1
power line cycle.
|
| OR
| 4 This is the 4 1/2 digit display mode.
4 [a] This mode provides 59 db NMR with
an integration time of 1 power line cy-
cle (16.66 mS at 60Hz, 20 mS at
50Hz).
5 The 5 1/2 digit display mode provides
fA]
the
best
resolution
and
greatest
amount of noise rejection. In this
mode, 10 readings are taken, each
with 1 power line cycle integration
time, and averaged together. This pro-
|
vides
80
db
of
noise
rejection.
on
|
|
Integration Times
As
mentioned
above,
changing
the
number
of
display
digits
does
more
than
merely
change
the
resolution
of
the
multimeter.
It
actually
changes
the
“INTEGRATION
TIME”,
which
determines
the
reading
rate.
This
in
turn
will
greatly
affect
the
Nor-
mal Mode Rejection (NMR) of the 3468A. The 3468A uses an integration
type of
A/D converter.
Integration
is a process where the effects of line related noise are
averaged to zero over the period of an integral number of power line cycles
(PLC’s)
during
an
A/D
conversion.
The
integration
time
is
not
the
same
as
the
time
for one measurement.
The integration
time is the time period, in PLC’s, during
which
the
input
voltage
is
sampled
by
the
voltmeter.
At
4
1/2
digit
display,
the
time
required
for
one
integration
cycle
is
one
PLC:
16.66
mS
at
6OHz
line fre-
quency, 20 mS for 50Hz. The integration
time is determined
by the setting of the
50/60Hz
line
switch
on the
rear
panel.
In
the
3
1/2
digit
mode,
the
integration
time
is
.1
PLC.
Normal
Mode
Rejection
(NMR)is
the
ability
of
a
voltmeter
to
ac-
curately
measure
dc
voltages
in
the
presence
of
ac
voltages
at
power
line
frequen-
cies. The 3468A has much better NMR at 4 1/2 digit display than it does at 3
1/2 digit display (59 db vs. Odb). The greatest amount of NMR is available from
the
5
1/2
digit
mode
(80
db)
where
ten
(10)
readings
are
taken
at
1
PLC
integra-
tion time and averaged together.
15