Chapter 2
Theory of Operation
©
National Instruments Corporation
2-21
The voltage reference source for each DAC is selectable from the onboard
reference or a voltage supplied at the EXTREF pin on the I/O connector,
except for the AT-MIO-16XE-50 and AT-MIO-16XE-10, which only
supports the onboard reference. The onboard reference is fixed at +10 V.
The external reference can be either a DC or an AC signal. If you apply an
AC reference, the analog output channel acts as a signal attenuator and the
AC signal appears at the output attenuated by the digital code. For unipolar
output the voltage is simply attenuated. Four quadrant multiplication
occurs in bipolar output, where the signal will not only be attenuated but
also inverted for negative digital codes.
The DAC output can be configured to produce either a unipolar or
bipolar output range, except for the AT-MIO-16XE-50, which supports
only bipolar output. A unipolar output has an output range of 0 to +Vref
–1 LSB V. A bipolar output has an output voltage range of -Vref to (+Vref
–1 LSB V). For unipolar output, the data written to the DAC is interpreted
in straight binary format. For bipolar output, the data is interpreted as two’s
complement format. One LSB is the voltage increment corresponding to an
LSB change in the digital code word. For unipolar output, 1 LSB =
(Vref)/4,096. For bipolar output, 1 LSB = (Vref)/2,048. For 16-bit DAC,
1 LSB = (Vref)/8,192 in unipolar mode and 1 LSB = (Vref)/4,096 in
bipolar mode.
Using the 12-bit DAC and onboard 10 V reference will produce an output
voltage range of 0 to 9.9976 V in steps of 2.44 mV for unipolar output and
an output voltage range of –10 V to +9.9951 V in steps of 4.88 mV for
bipolar operation. Using 16-bit DAC and onboard 10 V reference will
produce an output range of 0 to 9.9986 V in steps of 1.22 mV for unipolar
output and an output voltage range of –10 to +9.9976 V in steps of 2.44 mV
for bipolar operation.
In normal operation, a DAC output will glitch whenever it is updated with
a new value. The glitch energy differs from code to code and appears as
distortion in the frequency spectrum. Each analog output of the
AT-MIO-16E-1, AT-MIO-16E-2, and AT-MIO-64E-3 contains a reglitch
circuit that generates uniform glitch energy at every code rather than large
glitches at the major code transitions. This uniform glitch energy appears
as a multiple of the update rate in the frequency spectrum. Notice that this
reglitch circuit does not eliminate the glitches; it only makes them more
uniform in size.
The AT-MIO-16E-1, AT-MIO-16E-2, AT-MIO-64E-3, and
AT-MIO-16XE-10 include 2 kword-deep FIFOs to buffer the analog output
data. This buffering will increase the maximum rate that the analog output
Содержание AT-AI-16XE-10
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