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Chapter 6 Theory of Analog Operation
PCI-4451/4452 User Manual 6-6
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National Instruments Corporation
Figure 6-2.
Input Frequency Response Near the Cutoff
Because the ADC samples at 128 times the data rate, frequency
components above 64 times the data rate can alias. The digital filter rejects
most of the frequency range over which aliasing can occur. However, the
filter can do nothing about components that lie close to 128 times the data
rate, 256 times the data rate, and so on, because it cannot distinguish these
components from components in the baseband (0 Hz to the Nyquist
frequency). If, for instance, the sample rate is 200 kS/s and a signal
component lies within 100 kHz of 25.6 MHz (128 × 200 kHz), this signal
is aliased into the passband region of the digital filter and is not attenuated.
The purpose of the analog filter is to remove these higher frequency
components near multiples of the oversampling rate before they get to the
sampler and the digital filter.
While the frequency response of the digital filter scales in proportion to the
sample rate, the frequency response of the analog filter remains fixed. The
response of the filter is optimized to produce good high-frequency alias
rejection while having a flat in-band frequency response. Because this filter
is third order, its roll-off is rather slow. This means that, although the filter
has good alias rejection for high sample rates, it does not reject as well at
lower sample rates. The alias rejection near 128 times the sample rate
Amplitude (dB)
Frequency/Sample Rate (f
s
)
0.00
–1.00
–2.00
–3.00
–4.00
–5.00
–6.00
0.43 0.44 0.45 0.46 0.47 0.48 0.49 0.50
User.book Page 6 Tuesday, April 14, 1998 10:20 AM