Simultaneous Sample-And-Hold Modules; Delta-Sigma Modules; Slow Sample Rate Modules - National Instruments cRIO-904 Series User Manual

Simultaneous Sample-and-Hold Modules

Simultaneous sample-and-hold (SSH) C Series analog input modules contain multiple A/D
converters or circuitry that allows all the input channels to be sampled at the same time. These
modules sample their inputs on every Sample Clock pulse.

Delta-Sigma Modules

Delta-sigma C Series analog input modules function much like SSH modules, but use A/D
converters that require a high-frequency oversample clock to produce accurate, synchronized
data. Some delta-sigma modules in the cRIO controller automatically share a single
oversample clock to synchronize data from all the modules that support an external
oversample clock timebase when they all share the same task. (DSA modules are an example).
The oversample clock is used as the AI Sample Clock Timebase. The cRIO controller supplies
10 MHz, 12.8 MHz, and 13.1072 MHz timebases from which software automatically selects
based on the modules in the task. When delta-sigma modules with different oversample clock
frequencies are used in an analog input task, the AI Sample Clock Timebase can use any of the
available frequencies; by default, the fastest available is used. The sample rate of all modules
in the task is an integer divisor of the frequency of the AI Sample Clock Timebase.
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 controller automatically issues a
synchronization pulse to each delta-sigma module so that their ADCs are reset at the same
time. 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 channels from delta-sigma C Series modules are included in a multi-chassis task, please
ensure that the first channel in your channel list is from a delta-sigma module.
Note
DSA modules do not support HWTSP mode.

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 cRIO controller mixed with a non-slow sample module in the same task,
exceeding the maximum sampling rate of the slow sample module results in the most recently
acquired sample being read multiple times. In this scenario, the first sample of a hardware-
timed acquisition with a slow sampled C Series module is sampled when the task is
committed.
cRIO-904x User Manual | © National Instruments | 53