Data Translation DT9816 Series, Руководство пользователя

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Principles of Operation
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Note: 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
2 kHz signal, specify a sampling frequency of at least 4 kHz. Doing so avoids an error
condition called
aliasing , in which high frequency input components erroneously appear as
lower frequencies after sampling.
When you specify the sampling frequency of the internal clock, the driver determines the
actual frequency that the module can achieve by dividing the effective base clock frequency by
an internal clock divider. On the DT9816 and DT9816-A modules, the driver divides the base
frequency of 48 MHz by 12, which results in an effective base frequency of 4 MHz. On the
DT9816-S and DT9816-S-WTR module, the driver divides the base frequency of 48 MHz by 4,
which results in an effective base frequency of 12 MHz. The effective base clock frequency is
then divided by an internal clock divider.
It is possible that the actual sampling rate that is configured is different from the value that
you specified. For example, assume that you requested a sampling frequency of 42 kHz on the
DT9816-S module. In this example, 12 MHz / 42 kHz results in a clock divider of 285.71. Since
only the integer portion of the clock divider is used, the actual sampling frequency is
determined by dividing the base clock by the integer portion of the clock divider. In this
example, 12 MHz / 285 yields an actual sampling frequency of 42.105263 kHz; this is the
frequency that the module uses.
External A/D Sample Clock
An external A/D sample clock is useful when you want to pace acquisitions at rates not
available with the internal A/D sample clock or when you want to pace at uneven intervals.
Connect an external A/D sample clock to screw terminal 39 on the DT9816 Series modules or
to pin 19 on connector J4 of the DT9816-OEM module.
Using software, specify the clock source as external. For DT9816 Series modules, the clock
frequency is always equal to the frequency of the external A/D sample clock input signal that
you connect to the module.
Note: The external clock signal has a 22.1 k
Ω
pull-up to +5 V. Therefore, when this pin is
unconnected, the signal goes to +5 V.
Analog Input Conversion Modes
DT9816 Series modules support the following conversion modes:
• Single-value operations
• Continuous scan operations
The following subsections describes the conversion modes in more detail.