Data Translation DT9834 Series, Руководство пользователя

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Chapter 5
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Input Sample Clock Sources
DT9834 Series modules allow you to use one of the following clock sources to pace analog
input operations:
• Internal A/D clock – Using software, specify the clock source as internal and the clock
frequency at which to pace the operation. The minimum frequency supported is 0.75
Samples/s; the maximum frequency supported is 500 kSamples/s.
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 kHz signal, specify a sampling frequency of at least 40 kHz. Doing so avoids an error
condition called
aliasing , in which high frequency input components erroneously appear
as lower frequencies after sampling.
• External A/D clock – An external A/D clock is useful when you want to pace acquisitions
at rates not available with the internal A/D clock or when you want to pace at uneven
intervals.
Connect an external A/D clock to the External ADC Clock input signal on the DT9834
Series module. Conversions start on the falling edge of the external A/D clock input
signal.
Using software, specify the clock source as external. 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: If you specify channel 16 or 32 (the digital input port) and/or channels 17 through 26
or channels 33 through 42 (the counter/timer channels) in the channel list, the input sample
clock (internal or external) also paces the acquisition of the digital input port and/or
counter/timer channels.
Analog Input Conversion Modes
DT9834 Series modules support the following conversion modes:
• Single-value operations are the simplest to use. Using software, you specify the range,
gain, and analog input channel. The module acquires the data from the specified channel
and returns the data immediately. For a single-value operation, you cannot specify a clock
source, trigger source, scan mode, or buffer.
Single-value operations stop automatically when finished; you cannot stop a single-value
operation.
• Scan mode takes full advantage of the capabilities of the DT9834 Series modules. For a
scan, you can specify a channel list, clock source, trigger source, scan mode, and buffer
using software. Two scan modes are supported: continuous scan mode and triggered scan
mode (often called burst mode). These modes are described in the following subsections.
Using software, you can stop a scan by performing either an orderly stop or an abrupt
stop. In an orderly stop, the module finishes acquiring the data, stops all subsequent
acquisition, and transfers the acquired data to host memory; any subsequent triggers are
ignored.