3-4
|
ni.com
Chapter 3
Analog Input
When one or more delta-sigma modules are in an analog input task, the delta-sigma modules also
provide the signal used as the AI Sample Clock. This signal is used to cause A/D conversion for
other modules in the system, just as the AI Sample Clock does when a delta-sigma module is not
being used.
When delta-sigma modules are in an AI task, the chassis automatically issues a synchronization
pulse to each delta-sigma modules that resets their ADCs at the same time. You can also specify
a specific time for the sync pulse to occur. Because of the filtering used in delta-sigma A/D
converters, these modules usually exhibit a fixed input delay relative to non-delta-sigma
modules in the system. This input delay is specified in the C Series module documentation.
When using multichassis device tasks across multiple chassis that contains channels from
delta-sigma C Series modules ensure that one of those channels is the first in your channel list.
Slow Sample Rate Modules
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 3-2.
Figure 3-2.
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.
For more information about which C Series modules are compatible with the cDAQ chassis, go
to
ni.com/info
and enter the Info Code
rdcdaq
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