Interval Data Acquisition; Data Acquisition Rates - National Instruments DAQCard-1200 User Manual

Data Acquisition Rates

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Interval Data Acquisition

In an interval data acquisition operation, a new period called the interval
cycle is defined. Within each interval cycle, a predefined number of
samples is taken at regular sample intervals and conversions are stopped for
the remainder of the interval cycle. A data acquisition operation consists of
back-to-back interval cycles. In single-channel interval data acquisition,
you select the number of samples you want in each interval cycle from a
single channel. In multiple-channel interval data acquisition, the channels
are scanned once during each interval cycle. The interval cycle timing can
be externally controlled through OUTB1 or controlled internally by
counter B1 of the timing circuitry.
For an example of a multiple-channel interval data acquisition operation,
refer to the description of interval scanning signal timing in the
Timing Connections section in Chapter 3,
Maximum data acquisition rates (number of samples per second) are
determined by the conversion period of the ADC plus the sample-and-hold
acquisition time. During multiple-channel scanning, the data acquisition
rates are further limited by the settling time of the input multiplexers and
programmable gain amplifier. After the input multiplexers are switched,
the amplifier must be allowed to settle to the new input signal value to
within 12-bit accuracy before you perform an A/D conversion, or 12-bit
accuracy will not be achieved. The settling time is a function of the gain
selected.
Table 4-1 shows the recommended settling time for each gain setting
during multiple-channel scanning. Table 4-2 shows the maximum
recommended data acquisition rates for both single-channel and
multiple-channel data acquisition. For single-channel scanning, this rate is
limited only by the ADC conversion period plus the sample-and-hold
acquisition time, which is specified at 10 s (11 s if EXTCONV* is used).
Signal
4-9
Chapter 4 Theory of Operation
DAQ
Connections.
DAQCard-1200 User Manual