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The PCI-1712/1712L can adopt both internal and external clock sources
for pacer, post-trigger, delay-trigger, about-trigger acquisition modes:
Internal A/D sample clock with 16-bit Counter
External A/D sample clock that is connected to AI_CLK on the
PCLD-8712 screw terminal board.
The internal and external A/D sample clocks are described in more
detail as follows.
Internal A/D Sample Clock
The internal A/D sample clock uses a 10 MHz time base. Conversions
start on the rising 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 152.6 S/s, the maximum
frequency is 2 MS/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 A/D Sample Clock
The 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 AI_CLK on the PCLD-8712 screw
terminal board. Conversions will start on the rising edge of the external
A/D sample clock input signal. You can use software to specify the
clock source as external. The sampling frequency is always limited to a
maximum of 2 MHz for the external A/D sample clock input signal.
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Acquired number of samples N
2
3
Figure 5-4: Pre-Trigger Acquisition Mode
– 37 –
Trigger Event
2 Samples
N
N+1 N+2
PCI-1712/1712L User's Manual
Chapter 5
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