National Instruments DAQ X Series, Руководство пользователя

Страница: 55 / 299

Chapter 4 Analog Input
© National Instruments 4-9 X Series User Manual
For example, s u ppose all channels in a system u se a –5 V to 5 V
inp u t range. The signals on channels 0, 2, and 4 vary between
4.3 V and 5 V. The signals on channels 1, 3, and 5 vary between
–4 V and 0 V. Scanning channels in the order 0, 2, 4, 1, 3, 5
prod
u ces more acc u rate res u lts than scanning channels in the
order 0, 1, 2, 3, 4, 5.
4. 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. Here are two examples to consider:
• 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 500 points from each
channel at a scan rate of 250 kS/s. Another method wo u ld be to
acq u ire 1,000 points from each channel at a scan rate of 500 kS/s.
Both methods take the same amo u nt of time. Do u bling the n u mber
of samples averaged (from 500 to 1,000) 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 4
μ
s to 2
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 less often and is affected
m u ch 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.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com