National Instruments Data Acquisition Device E Series - Manual (Page 46)

Chapter 2 Analog Input
© National Instruments Corporation 2-13 E Series 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 and then read
100 points from channel 1. The second method switches between channels
much less often and is affected much less by settling time.
AI Data Acquisition Methods
When performing analog input measurements, there are several different
data acquisition methods available. You can either 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 point of data.
Hardware-Timed Acquisitions
With hardware-timed acquisitions, a digital hardware signal 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 can be deterministic.
Hardware-timed acquisitions can use hardware triggering.
Hardware-timed operations can be buffered or non-buffered. A buffer is a
temporary storage in the computer memory where acquired samples are
stored.