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Chapter 2 Analog Input

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-9184/9188) 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 cDAQ 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 C Series Support in NI-DAQmx for more information. To access this document, go to

ni.com/info and enter the Info Code rdcdaq.

AI Start Trigger Signal

Use the Start Trigger signal to begin a measurement acquisition. A measurement acquisition

consists of one or more samples. If you do not use triggers, begin a measurement with a software

command. Once the acquisition begins, configure the acquisition to stop in one of the following

ways:

• When a certain number of points has been sampled (in finite mode)

• After a hardware reference trigger (in finite mode)

• With a software command (in continuous mode)

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

AA ABB BC

1 2 ... 41 42 43 44 45 46 47 48 49 50 51 ... 126 127

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WHITE BROWN DA 918 Manual de usuario Pagina 46