Table of Contents
Advertisement
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?
Advertisement
Table of Contents
