Data Translation DT9828, User Manual

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Chapter 5
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Note: If you are continuously measuring from a properly configured thermocouple input
channel and the thermocouple opens or becomes disconnected, the open thermocouple
pull-up circuit causes the input voltage to rise to approximately 1.5 V over a few seconds.
In this case, the temperature value rises very quickly, and you will receive the
TEMP_OUT_OF_RANGE_HIGH (88888 decimal) value followed by the OPEN_SENSOR
(99999 decimal) value.
Input Resolution
The input resolution of the analog input subsystem on the DT9828 module is 24 bits.
A/D Sample Clock
The DT9828 module uses an internal A/D sample clock with a time base of 4.9152 MHz to
pace analog input operations in continuous mode. This clock paces the acquisition of each
channel in the channel list.
The maximum total throughput is 600 Samples/s. The minimum total throughput rate is
0.5865 Samples/s.
The data rate per channel is computed as follows:
Data Rate per Channel = Throughput Rate/Number of Channels
For example, to sample all eight channels at the maximum rate, the data rate per channel is
600/8, or 75 Samples/s.
Using software, specify the channels that you want to sample in the channel list, and then set
the desired sampling frequency. The software sets the sampling frequency to the closest
possible value. You can use software to return the actual frequency that is used.
Note: In QuickDAQ, you specify the per channel sampling frequency. In the DT-Open
Layers Class Library and the Data Acq SDK, you specify the aggregate (total) sampling
frequency. See
Table 2 for examples of the per channel sampling frequency based on the
number of channels.
According to sampling theory (Nyquist Theorem), specify a frequency that is at least twice as
fast as the input’s highest frequency component. For example, to accurately sample a 20 Hz
signal, specify a sampling frequency of at least 40 Hz. Doing so avoids an error condition
called
aliasing , in which high frequency input components erroneously appear as lower
frequencies after sampling.