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Chapter 4
Analog Input
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.
NI 6238/6239 User Manual
When this calculation results in the sampling rate exceeding 35 kHz, there
is not enough time between samples to acquire both channels and still add
a 10 μs delay per channel, so the Convert Clock rate becomes the sampling
rate multiplied by the number of channels being acquired. For example, on
the PCI-6220 M Series device, a sampling rate of 40 kHz for two channels
would result in a Convert Clock rate of 80 kHz.
Maximum settling time for the amplifier is also very important. For
example, to ensure accuracy to within ± 1 LSB on an NI 623x M Series
device, the device requires a minimum amplifier settling time of 6 μs even
though the maximum channel conversion rate is 4 μs. Higher source
impedance also increases amplifier settling time.
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 4-9. In this example, the DAQ device reads two channels
five times. 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.
ai/StartTrigger
ai/SampleClock
ai/ConvertClock
Sample Counter
Figure 4-9. 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 4-10 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
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