Data Translation DT9824, User Manual, Page: 51 / 102

Principles of Operation
51
As a guideline, the sampling frequency of the DT9824 should be a least 2.5 times the highest
input signal frequency that you are trying to measure. For example, to accurately sample a
400 Hz signal, it is recommended that you specify a sampling frequency of at least 1000 Hz.
The actual frequency that the module can achieve may be slightly different than the frequency
you specified due to the accuracy of the clock. You can determine the actual clock frequency
using software.
Triggers
A trigger is an event that occurs based on a specified set of conditions. The DT9824 module
supports the following trigger sources:
• Software trigger – A software trigger event occurs when you start the analog input
operation (the computer issues a write to the module to begin conversions). Using
software, specify the trigger source as a software trigger.
• External trigger – An external digital trigger occurs when the DT9824 module detects
either a falling-edge or rising-edge on the External trigger line. The edge-type is
software-programmable.
The external trigger connection is a differential input, allowing for TTL-level or switch
connections. The positive side is pulled up to +5 V. If you are using the external trigger to
detect a contact closure, ensure that you configure the polarity of the external trigger for a
falling-edge (high-to-low) transition.
Analog Input Conversion Modes
The DT9824 module supports the following conversion modes:
• Single-value operations are the simplest to use but offer the least flexibility and
efficiency. Using software, you can specify the range, gain, and index of the analog input
channel, and acquire the data from that channel. The data is returned immediately. For a
single-value operation, you cannot specify a clock source, trigger source, or buffer.
Single-value operations stop automatically when finished; you cannot stop a single-value
operation.
• Continuous mode takes full advantage of the capabilities of the DT9824 module. You can
specify a trigger source and buffer using software. (Refer to page 53 for more information
on buffers.)
You can stop a continuous operation by performing either an orderly stop or an abrupt
stop using software. In an orderly stop, the module finishes acquiring the data, stops all
subsequent acquisition, and transfers the acquired data to host memory; all subsequent
triggers are ignored. In an abrupt stop, the module stops acquiring samples immediately;
the acquired data is not transferred to host memory, and all subsequent triggers or
retriggers are ignored.
The conversion rate is determined by the frequency of the A/D sample clock; refer to page
50 for more information on the A/D sample clock.
DT9824 modules use an 8 kSample input FIFO for storing data from each of up to 5 enabled
input channels (analog input indices 0 to 3, and the digital input port, index 4).