National Instruments Data Acquisition Device NI USB-621x, User Manual

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Chapter 4 Analog Input
NI USB-621x User Manual 4-8 ni.com
• Avoid Scanning Faster Than Necessary —Designing yo u r system to
scan at slower speeds gives the NI-PGIA more time to settle to a more
acc
u rate level. Consider the following examples:
– Example 1—Averaging many AI samples can increase the
acc
u racy of the reading by decreasing noise effects. In general,
the more points yo u average, the more acc u rate the final res u lt.
However, yo u may choose to decrease the n u mber of points yo u
average and slow down the scanning rate.
S u ppose yo u want to sample 10 channels over a period of 20 ms
and average the res u lts. Yo u co u ld acq u ire 250 points from each
channel at a scan rate of 125 kS/s. Another method wo u ld be to
acq u ire 500 points from each channel at a scan rate of 250 kS/s.
Both methods take the same amo u nt of time. Do u bling the n u mber
of samples averaged (from 250 to 500) decreases the effect of
noise by a factor of 1.4 (the sq
u are root of 2). However, do u bling
the n u mber of samples (in this example) decreases the time the
NI-PGIA has to settle from 8
μ
s to 4
μ
s. In some cases, the slower
scan rate system ret u rns more acc u rate res u lts.
– Example 2—If the time relationship between channels is not
critical, yo
u can sample from the same channel m u ltiple times and
scan less freq u ently. For example, s u ppose an application req u ires
averaging 100 points from channel 0 and averaging 100 points
from channel 1. Yo
u co u ld alternate reading between
channels—that is, read one point from channel 0, then one point
from channel 1, and so on. Yo
u also co u ld read all 100 points from
channel 0 then read 100 points from channel 1. The second
method switches between channels m
u ch often and is affected less
by settling time.
Analog Input Data Acquisition Methods
When performing analog inp u t meas u rements, yo u either can perform
software-timed or hardware-timed acq u isitions:
• Software-Timed Acquisitions —With a software-timed acq u isition,
software controls the rate of the acq u isition. Software sends a separate
command to the hardware to initiate each ADC conversion. In
NI-DAQmx, software-timed acq
u isitions are referred to as having
on-demand timing. Software-timed acq u isitions are also referred to as
immediate or static acq u isitions and are typically u sed for reading a
single sample of data.