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© National Instruments Corp. 15 NI 9235/9236
Understanding NI 9235/9236 Filtering
The NI 9235/9236 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 9235/9236 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.
Passband
The signals within the passband have frequency-dependent gain or
attenuation. The small amount of variation in gain with respect to
frequency is called the passband flatness. The digital filters of the
NI 9235/9236 adjust the frequency range of the passband to match
the data rate. Therefore, the amount of gain or attenuation at a
given frequency depends on the data rate. Figure 7 shows typical
passband flatness for the 10 kS/s data rate.