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Chapter 6 Theory of Analog Operation
PCI-4451/4452 User Manual 6-4
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National Instruments Corporation
has an extremely flat frequency response in the bandwidth of interest, and
it has very little phase error.
The analog filter precedes the analog sampler, which operates at 128 times
the selected sample rate (26.2144 MS/s in the case of a 204.8 kS/s sample
rate) and is actually a 1-bit ADC. The 1-bit, 128-times oversampled data
that the analog sampler produces is passed on to a digital antialiasing filter
that is built into the ADC chip. This filter also has extremely flat frequency
response and no phase error, but its roll-off near the cutoff frequency (about
0.493 times the sample rate) is extremely sharp, and the rejection above
0.536 times the sample rate is greater than 85 dB. The output stage of the
digital filter resamples the higher frequency data stream at the output data
rate, producing 16-bit digital samples.
With the PCI-4451/4452 filters, you have the complete antialiasing
protection needed to sample signals accurately. The digital filter in each
channel passes only those signal components with frequencies that lie
below the Nyquist frequency or within one Nyquist bandwidth of multiples
of 128 times the sample rate. The analog filter in each channel rejects
possible aliases (mostly noise) from signals that lie near these multiples.
Figures 6-1 and 6-2 show the frequency response of the PCI-4451/4452
input circuitry.
User.book Page 4 Tuesday, April 14, 1998 10:20 AM