Chapter 4 Analog Input
NI USB-621x User Manual 4-12 ni.com
Figure 4-5. Interval Sampling
AI Convert Clock controls the Convert Period, which is determined by the
following equation:
1/Convert Period = Convert Rate
Note The sampling rate is the fastest you can acquire data on the device and still achieve
accurate results. For example, if a USB-621x 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 illustrates the relationship.
Posttriggered data acquisition allows you to view only data that is acquired
after a trigger event is received. A typical posttriggered DAQ sequence
is shown in Figure 4-6. The sample counter is loaded with the specified
number of posttrigger samples, in this example, five. The value decrements
with each pulse on AI Sample Clock, until the value reaches zero and all
desired samples have been acquired.
Figure 4-6. Posttriggered Data Acquisition Example
Channel 0
Channel 1
Convert Period
Sample Period
1 3 0 4 2
AI Start Trigger
AI Sample Clock
AI Convert Clock
Sample Counter
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