Chapter 2
Theory of Operation
2-10
©
National Instruments Corporation
decreases noise modulation and improves differential linearity. Dither
should be disabled for high-speed applications not involving averaging
because it would only add noise. When taking DC measurements, such as
when calibrating the board, you should enable dither and average about
1,000 points to take a single reading. This process removes the effects of
quantization, reduces measurement noise, and improves resolution.
Notice that dither cannot be disabled on the AT-MIO-16XE-50,
AT-MIO-16XE-10, and AT-AI-16XE-10.
The last channel bit is used to indicate that this is the last conversion in a
scan. The DAQ-STC will end the scan on the conversion with this bit set.
The AT E Series boards use sampling, successive approximation ADCs
with 12 or 16 bits of resolution with maximum conversion rates between
50
µ
s and 800 ns. The converter can resolve its input range into 4,096
different steps for the 12-bit ADC and 65,536 for the 16-bit ADC. The input
range of the 12-bit boards is ±5 V in bipolar mode and 0 to +10 V in
unipolar mode. These modes correspond to ranges of –2,048 to 2,047 in
unipolar mode and 0 to 4,095 in bipolar mode. The input range of the 16-bit
boards is ±10 V in bipolar mode and 0 to +10 V in unipolar mode. These
modes correspond to ranges of –32,768 to 32,767 in bipolar mode and 0 to
65,535 in unipolar mode.
The AT E Series boards include a 16-bit wide FIFO to buffer the analog
input data. This buffering will increase the maximum rate that the analog
input can sustain during continuous acquisition. The FIFO is 8 kwords deep
on the AT-MIO-16E-1, 2 kwords deep on the AT-MIO-16E-2 and
AT-MIO-64E-3, and 512 words deep on the others. The DAQ-STC shifts
the data into the FIFO from the ADC when the conversion is complete. This
buffering allows the ADC to begin a new conversion even though the data
has not yet been read from the board. This buffering also provides more
time for the software or DMA to respond and read the analog input data
from the board. If the FIFO is full and another conversion completes, an
error condition called FIFO overflow occurs and the data from that
conversion is lost. The FIFO not empty, half-full, and full flags are
available to generate interrupts or DMA requests for the data transfer.
Measurement reliability is assured through the onboard calibration
circuitry of the board. This circuitry uses an internal, stable 5 V reference
that is measured at the factory against a higher accuracy reference; its
value is then stored in the EEPROM. With this stored reference value,
the board can be recalibrated at any time under any number of different
environmental conditions in order to remove errors caused by time and
temperature drift. The EEPROM stores calibration constants that can be
Содержание AT-AI-16XE-10
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