National Instruments NI USB-621x - Manual (Page 39)

Chapter 4 Analog Input
© National Instruments Corporation 4-13 NI USB-621x User Manual
NI-DAQmx chooses the default convert rate to allow for the maximum
settling time between conversions. Typically, this rate is the sampling rate
for the task multiplied by the number of channels in the task.
Note The sampling rate is the fastest you can acquire data on the device and still achieve
accurate results. For example, if an M Series 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/SampleClock, until the value reaches zero and all
desired samples have been acquired.
Figure 4-6. Posttriggered Data Acquisition Example
Pretriggered data acquisition allows you to view data that is acquired before
the trigger of interest, in addition to data acquired after the trigger.
Figure 4-7 shows a typical pretriggered DAQ sequence. ai/StartTrigger can
be either a hardware or software signal. If ai/StartTrigger is set up to be a
software start trigger, an output pulse appears on the ai/StartTrigger line
when the acquisition begins. When the ai/StartTrigger pulse occurs, the
sample counter is loaded with the number of pretriggered samples, in this
example, four. The value decrements with each pulse on ai/SampleClock,
until the value reaches zero. The sample counter is then loaded with the
number of posttriggered samples, in this example, three.
1 3 0 4 2
ai/StartTrigger
ai/SampleClock
ai/ConvertClock
Sample Counter