Chapter 3
Device Overview and Theory of Operation
©
National Instruments Corporation
3-5
This process by which the sampler modulates out-of-band components
back down the Nyquist bandwidth is known as aliasing. The greatest danger
of aliasing is that there is no straightforward way to know whether it has
happened by looking at the ADC output. The 25 Hz observation described
above will look just like a real 25 Hz signal in the digital domain, but it is
clearly nowhere close the true analog frequency of 3,975 Hz. If an input
signal contains several frequency components or harmonics, it is quite
possible that some of these components will be represented correctly while
others will be aliased.
Lowpass filtering to eliminate all components above the Nyquist frequency
either before or during the digitization process can guarantee that the
digitized data set is free of all aliases. The NI 447
X
employs both digital
and analog low-pass filters to achieve this end.
The delta-sigma ADCs on the NI 447
X
include brick-wall digital filters
whose cut-off frequency tracks the sampling rate. Thus, the filter topology
automatically adjusts to follow the Nyquist frequency. The cutoff
frequency of the digital filters is 0.4863
f
s
. The rejection for frequency
components above 0.5465
f
s
is greater than 110 dB. In the passband below
0.4535
f
s
, the amplitude response of the filter is extremely flat. The digital
filter has a negligible effect on frequency components that lie in the band
of interest. Because the filter employs an FIR (Finite Impulse Response)
architecture, its phase response is perfectly linear. Figures 3-4 and 3-5
show the frequency response of the NI 447
X
input circuitry.
