Chapter 2
Using the Module
©
National Instruments Corporation
2-27
Analog Input Filters
The NI PXIe-4330/4331 uses a combination of analog and digital filtering to provide an
accurate representation of in-band signals while rejecting out-of-band signals. The filters
discriminate between signals based on the frequency range, or bandwidth, of the signal. The
three important bandwidths to consider are the passband, the stopband, and the alias-free
bandwidth.
The NI PXIe-4330/4331 accurately represents signals within the passband, as quantified
primarily by passband flatness and phase nonlinearity. 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 whether 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.
Lowpass 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. The NI PXIe-4330/4331 modules employ both digital and analog lowpass filters
to achieve this protection.
The NI PXIe-4330/4331 modules include an oversampled architecture and sharp digital filters
with cut-off frequencies that track the sampling rate. Thus, 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.
In addition to the digital filtering, the NI PXIe-4330/4331 modules feature a fixed-frequency
analog filter. The 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.
While the frequency response of the digital filter directly scales with the sample rate, the
analog filter –3dB point is fixed. The NI PXIe-4330/4331 automatically adjusts its
oversample rate to maintain a high level of alias protection regardless of the current sampling
rate.
