Data Translation DT9812 Series, Руководство пользователя

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Principles of Operation
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Input Sample Clock Sources
You can use one of the following clock sources to pace an analog input operation:
• Internal clock – Using software, specify the clock source as internal and the clock
frequency at which to pace the operation.
For the DT9812-2.5V, DT9812-10V, DT9812-10V-OEM, DT9813-10V, and DT9814-10V, the
minimum frequency of the internal clock is 30 Hz; the maximum frequency of the internal
clock is 50 kHz.
For the DT9812A, DT9813A, and DT9814A, the minimum frequency of the internal clock
is 30 Hz; the maximum frequency of the internal clock is 100 kHz.
Because these modules have a multiplexed A/D architecture, if you specify two analog
input channels in the channel list, the maximum sampling rate per channel is divided by
the number of channels in the channel list. For example, if the maximum sample rate for
the module is 50 kSamples/s, and you have two channels in the channel list, the
maximum sample rate is 25 kSamples/s for each channel. Likewise, if the maximum
sample rate for the module is 100 kSamples/s, and you have two channels in the channel
list, the maximum sample rate is 50 kSamples/s for each channel. Further, if you specify
16 analog input channels in the channel list, the maximum sampling rate is
3.125 kSamples/s for each channel (if the maximum sample rate is 50 kSamples/s for the
device) or 6.25 kSamples/s for each channel (if the maximum sample rate is
100 kSamples/s for the device).
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.
• External clock – An external clock is useful when you want to pace acquisitions at rates
not available with the internal clock or when you want to pace at uneven intervals. The
minimum frequency of the external clock can be less than 30 Hz. For the DT9812-2.5V,
DT9812-10V, DT9812-10V-OEM, DT9813-10V, and DT9814-10V, the maximum frequency
of the external clock is 40 kHz. For the DT9812A, DT9813A, and DT9814A, the maximum
frequency of the external clock is 75 kHz.
Connect an external clock to the Ext Clock In signal on the module. Conversions start on
the rising edge of the external clock input signal.
Using software, specify the clock source as external. The clock frequency is always equal
to the frequency of the external sample clock input signal that you connect to the module.
Note: To avoid overrun and underrun errors, it is recommended that you do not run both
the A/D and D/A subsystems at the same time using the maximum frequencies.