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Analog Input Features
2-17
analog input, the internal clock source can serve to pace other events
such as the digital I/O through the use of interrupts.
●
Hardware (external clock source)
— The external pacer clock
source must be an externally applied TTL-compatible signal attached
to XPCLK (pin 44 of the main I/O connector or pin 38 of STA-1800U
connectors J1 and J2). The active edge for this clock is
software-selectable.
An external clock source is useful if you want to pace at rates not
available with the 82C54 counter/timer, if you want to pace at uneven
intervals, or if you want to pace on the basis of an external event. An
external clock also allows you to synchronize multiple boards with a
common timing source.
Note:
The ADC acquires samples at a maximum of 333 ksamples/s (one
sample every 3.0µs) for DAS-1800ST Series boards and 100 ksamples/s
(one sample every 10µs) for the DAS-1800HR Series. If you are using an
external clock, make sure that it does not initiate conversions at a faster
rate than the ADC can handle.
If you are acquiring samples from multiple channels, the maximum
sampling rate for each channel is equal to the maximum allowable
conversion rate divided by the number of channels (see “Maximum
Achievable Throughput Rates” on page 2-9).
Burst Mode Conversion Clock
In burst mode and burst mode with SSH, the burst mode conversion clock
determines the conversion rate. (The burst mode conversion clock is not
used for paced mode.)
The burst mode conversion clock frequency is programmable for a range
of 15.625kHz to 333kHz (64µs to 3µs in 1µs increments) for the
DAS-1800ST Series boards and 15.625kHz to 100kHz (64µs to 10µs in
1µs increments) for the DAS-1800HR Series boards.
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