Chapter 4
Analog Input
4-12
ni.com
Figure 4-5.
Interval Sampling
AI Convert Clock controls the Convert Period, which is determined by the
following eq
u
ation:
1/Convert Period = Convert Rate
Note
The sampling rate is the fastest yo
u
can acq
u
ire data on the device and still achieve
acc
u
rate res
u
lts. For example, if a USB-621
x
device has a sampling rate of 250 kS/s, this
sampling rate is aggregate—one channel at 250 kS/s or two channels at 125 kS/s per
channel ill
u
strates the relationship.
Posttriggered data acq
u
isition allows yo
u
to view only data that is acq
u
ired
after a trigger event is received. A typical posttriggered DAQ seq
u
ence
is shown in Fig
u
re 4-6. The sample co
u
nter is loaded with the specified
n
u
mber of posttrigger samples, in this example, five. The val
u
e decrements
with each p
u
lse on AI Sample Clock,
u
ntil the val
u
e reaches zero and all
desired samples have been acq
u
ired.
Figure 4-6.
Posttriggered Data Acquisition Example
Ch
a
nnel 0
Ch
a
nnel 1
Convert Period
Sa
mple Period
1
3
0
4 2
AI
S
t
a
rt Trigger
AI
Sa
mple Clock
AI Convert Clock
Sa
mple Co
u
nter
