Ai Sample Clock Source Connections - Advantech MIC-1810 User Manual

3.8.2

AI Sample Clock Source Connections

Internal AI Sample Clock
The internal AI sample clock uses a 100 MHz time base. Conversions start on the ris-
ing edge of the counter output. You can use software to specify the clock source as
internal and the sampling frequency to pace the operation. The minimum frequency
is 0.024 S/s, the maximum frequency is 500 KS/s. According to the sampling theory
(Nyquist Theorem), you must specify a frequency that is at least twice as fast as the
input's highest frequency component to achieve a valid sampling. For example, to
accurately sample a 20 kHz signal, you have to specify a sampling frequency of at
least 40 kHz. This consideration can avoid an error condition often know as aliasing,
in which high frequency input components appear erroneously as lower frequencies
when sampling.
External AI Sample Clock
The external AI sample clock is useful when you want to pace acquisitions at rates
not available with the internal AI sample clock, or when you want to pace at uneven
intervals. Connect an external AI sample clock to screw terminal AI_CLK on the
screw terminal board. Conversions will start on the rising edge of the external AI
sample clock input signal. You can use software to specify the clock source as exter-
nal. The sampling frequency is always limited to a maximum of 10 MHz for the exter-
nal AI sample clock input signal.
Figure 3.9 External Clock Source Connection
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