National Instruments NI 447 Series User Manual page 37

Chapter 3 Device Overview and Theory of Operation
Although the digital filter eliminates almost all out-of-band components,
it is still susceptible to aliases from certain narrow frequency bands,
specifically those bands that lie within plus or minus one Nyquist
bandwidth of 64 × f > 51.2 kS/s) or 128 × f
(for f (for f < 51.2 kS/s).
s s s s
For example, if f = 10,000 S/s, the digital filter could admit aliases from
s
analog components between 1,275,000 Hz and 1,285,000 Hz.
In addition the digital filtering built in to the ADCs, the NI 447X devices
also feature a fixed-frequency analog filter. The analog filter serves to
remove high-frequency components in the analog signal path before they
reach the ADC. This addresses the possibility of any high-frequency
aliasing from the narrow bands not covered by the digital filter. Each input
channel on the NI 447X is equipped with a two-pole lowpass Butterworth
filter. The analog filter cutoff frequency is about 400 kHz. This filter has a
gradual rolloff with very little attenuation for any frequencies within the
45 kHz input bandwidth of the NI 447X. The high cutoff frequency of the
filter guarantees extremely flat amplitude response and minimal phase error
for signals of interest. Frequency components that could pass the digital
filter usually consist of high frequency noise. The analog lowpass filter
removes these high-frequency components before they ever reach
the ADC.
While the frequency response of the digital filter scales directly with the
sample rate, the response of the analog filter is fixed. The analog filter
response is optimized to produce good high-frequency alias rejection while
maintaining a flat in-band frequency response. Because the analog filter is
a two-pole system, its rolloff is not extremely sharp. It has excellent alias
rejection at higher sampling rates, where only very high frequencies could
pass through the digital filter. At lower sampling rates it does not filter
potential aliases perfectly, but in most cases these residual aliases are noise
rather than well-defined harmonics. Figure 3-6 shows the alias rejection
near 64 or 128 times the sample rate (bands passed by the digital filter)
versus sample rate. This graph accounts for the response of both the digital
and analog filters. The rejection ratio is better than 110 dB for all aliases
outside plus or minus one Nyquist bandwidth of multiples of 64 or
128 times f .
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© National Instruments Corporation 3-7 NI 447X User Manual