Chapter 3
Analog Output
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National Instruments Corporation
3-3
Polarity Selection
(NI 6020E, NI PXI-6030E, NI PCI-6031E, NI PXI-6040E, NI 6052E, PCI-MIO-16E-4,
and PCI-MIO-16XE-10 Devices Only)
With these devices, you can configure
each AO channel for either unipolar or bipolar output. All other E Series
devices are configured for bipolar output only. A unipolar configuration
has a range of 0 to V
ref
at the analog output. A bipolar configuration has a
range of –V
ref
to +V
ref
at the analog output. V
ref
is the voltage reference
used by the DACs in the AO circuitry and can be either the +10 V onboard
reference or for supported devices, an externally supplied reference within
±11 V. You do not need to configure both channels for the same range.
Selecting a bipolar range for a particular DAC means that any data written
to that DAC is interpreted as two’s complement format. In two’s
complement format, data values written to the AO channel can be either
positive or negative. If you select unipolar range, data is interpreted in
straight binary format. In straight binary mode, data values written to the
AO channel range must be positive.
Reglitch Selection
(NI 6052E and NI 6070E/6071E Devices Only)
In normal operation, a DAC
output glitches 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 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.
Reglitching is normally disabled at startup and the software can
independently enable each channel.
Minimizing Glitches on the Output Signal
When you use a DAC to generate a waveform, you may observe glitches
on the output signal. These glitches are normal; when a DAC switches from
one voltage to another, it produces glitches due to released charges. The
largest glitches occur when the most significant bit (MSB) of the DACcode
switches. You can build a lowpass deglitching filter to remove some of
these glitches, depending on the frequency and nature of the output signal.
Visit
ni.com/support
for more information about minimizing glitches.
