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Chapter 3
Signal Acquisition
done by the module, the sample rate resolution improves (becomes smaller) as the sample rate
decreases. If a sample rate is requested that the module is not capable of producing exactly,
NI-DAQmx software will automatically coerce the sample rate up to the next sample rate that
the module is capable of producing. The DAQmx Timing Property Node (
Sample Clock»Rate
)
can be used to read the actual sample rate used by hardware.
Frequency Domain Mode Acquisitions
In frequency domain mode, the PXIe-4480/4481 uses a combination of analog and digital
filtering to provide an accurate representation of in-band signals while rejecting out-of-band
signals. These filters discriminate between signals based on the frequency range, or bandwidth,
of the signal. The three important bands to consider are the passband, the stopband, and the
alias-free bandwidth. NI-DAQmx uses frequency domain mode automatically any time the
requested sample rate is at or below 1.25 MS/s. For sample rates above 1.25 MS/s, time domain
mode is always used.
The PXIe-4480/4481 accurately represents signals within the passband, as quantified primarily
by passband flatness. All signals that appear in the alias-free bandwidth are either unaliased
signals or signals that have been filtered by at least the amount of the stopband rejection.
Anti-Alias Filters
A digitizer or ADC might sample signals containing frequency components above the Nyquist
limit. The undesirable effect of the digitizer modulating out-of-band components into the
Nyquist bandwidth is aliasing. The greatest danger of aliasing is that you cannot determine if
aliasing occurred by looking at the ADC output. If an input signal contains several frequency
components or harmonics, some of these components might be represented correctly while
others contain aliased artifacts.
Low-pass filtering to eliminate components above the Nyquist frequency either before or during
the digitization process can guarantee that the digitized data set is free of aliased components. In
frequency domain mode, the PXIe-4480/4481 modules employ both digital and analog low-pass
filters to achieve this protection.
In frequency domain mode, the PXIe-4480/4481 modules use an oversampled architecture and
sharp digital filters with cut-off frequencies that track the sampling rate. Therefore the filter
automatically adjusts to follow the Nyquist frequency. Although the digital filter eliminates
almost all out-of-band components, it is still susceptible to aliases from certain narrow frequency
bands located at frequencies far above the sampling rate. These frequencies are referred to as the
ADC alias holes.
To minimize the error from the ADC alias holes, the PXIe-4480/4481 modules feature a
fixed-frequency analog filter. This analog filter removes high-frequency components in the
analog signal path before they reach the ADC. This filtering addresses the possibility of
high-frequency aliasing from the narrow bands that are not covered by the digital filter.