Ai Sample Clock Source Connections - Advantech MIC-1816 Series User Manual

However, the disadvantage of this connection is that the source is loaded with the
series combination (sum) of the two resistors. For ra and rb, for example, if the input
impedance rs is 1k Ohm, and each of the two resistors is 100k Ohm, then the resis-
tors load the signal source with 200 Ohm (100 Ohm + 100 Ohm), resulting in a 0.5%
gain error. Below is a simplified representation of the circuit and calculating process.
2.7.3

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. Users can set the clock source as internal and specify
the sampling frequency via software in order to pace operations. The minimum fre-
quency is 0.024 S/s, the maximum frequency is 1 MS/s. According to the sampling
theory (Nyquist Theorem), users must specify a frequency that is at least twice as
fast as the input's highest frequency component to achieve valid sampling. For exam-
ple, to accurately sample a 20 kHz signal, a sampling frequency of at least 40 kHz
must be specified. This consideration can avoid an error condition often know as
aliasing, in which high frequency input components appear erroneously as lower fre-
quencies when sampling.
External AI Sample Clock
The external AI sample clock is useful for pacing acquisitions at rates not available
with the internal AI sample clock, or pacing 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. Use software
to set the clock source as external.
The sampling frequency is always limited to a maximum of 10 MHz for the external AI
sample clock input signal.
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