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Chapter 2 Analog Input
filtering used in sigma-delta A/D converters, these modules usually exhibit a fixed input delay
relative to non-sigma-delta modules in the system. This input delay is specified in the C Series
I/O module documentation.
Slow Sample Rate Modules
(NI cDAQ-9174/9178) Some C Series analog input modules are specifically designed for
measuring signals that vary slowly, such as temperature. Because of their slow rate, it is not
appropriate for these modules to constrain the AI Sample Clock to operate at or slower than their
maximum rate. When using such a module in the NI cDAQ-9174/9178 chassis, the maximum
Sample Clock rate can run faster than the maximum rate for the module. When operating at a
rate faster than these slow rate modules can support, the slow rate module returns the same point
repeatedly, until a new conversion completes. In a hardware-timed task, the first point is
acquired when the task is committed. The second point is acquired after the start trigger as shown
in Figure 2-2.
Figure 2-2. Sample Clock Timing Example
For example, if running an AI task at 1 kHz using a module with a maximum rate of 10 Hz, the
slow module returns 100 samples of the first point, followed by 100 samples of the second point,
etc. Other modules in the task will return 1,000 new data points per second, which is normal.
When performing a single-point acquisition, no points are repeated. To avoid this behavior, use
multiple AI timing engines, and assign slow sample rate modules to a task with a rate at or slower
than their maximum rate.
Refer to the Developer Zone document, C Series Support in NI-DAQmx, for more information.
To access this Developer Zone document, go to
ni.com/info and enter the Info Code
rdcdaq.
StartTrigger
Data from
A/D Conversion
(Slow Module)
SampleClock
Data Returned
to AI Task
1st A/D Conversion 2nd A/D Conversion 3rd A/D Conversion
A
BC
AA ABB BC
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