National Instruments NI USB-621x - Manual (Page 35)

Chapter 4 Analog Input
© National Instruments Corporation 4-9 NI USB-621x User Manual
Example 2
If the time relationship between channels is not critical, you can sample
from the same channel multiple times and scan less frequently. For
example, suppose an application requires averaging 100 points from
channel 0 and averaging 100 points from channel 1. You could alternate
reading between channels—that is, read one point from channel 0, then
one point from channel 1, and so on. You also could read all 100 points
from channel 0 then read 100 points from channel 1. The second method
switches between channels much less often and is affected much less by
settling time.
Analog Input Data Acquisition Methods
When performing analog input measurements, you either can perform
software-timed or hardware-timed acquisitions. Hardware-timed
acquisitions can be buffered or non-buffered.
Software-Timed Acquisitions
With a software-timed acquisition, software controls the rate of the
acquisition. Software sends a separate command to the hardware to initiate
each ADC conversion. In NI-DAQmx, software-timed acquisitions are
referred to as having on-demand timing. Software-timed acquisitions are
also referred to as immediate or static acquisitions and are typically used
for reading a single sample of data.
Hardware-Timed Acquisitions
With hardware-timed acquisitions, a digital hardware signal
(ai/SampleClock) controls the rate of the acquisition. This signal can be
generated internally on your device or provided externally.
Hardware-timed acquisitions have several advantages over software-timed
acquisitions.
• The time between samples can be much shorter.
• The timing between samples is deterministic.
• Hardware-timed acquisitions can use hardware triggering.
Hardware-timed operations can be buffered or non-buffered.