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Chapter 3
Device Overview and Theory of Operation
©
National Instruments Corporation
3-5
If the signal in the previous example is not a pure sine wave, the signal can
have many components (harmonics) that lie above the Nyquist frequency.
If present, these harmonics are erroneously aliased back into the baseband
and added to the parts of the signal that are sampled accurately, producing
a distorted sampled data set. To avoid this, it is important to input to the
sampler only those signals that can be accurately represented—those
whose frequency components all lie below the Nyquist frequency. To make
sure that only those signals go into the sampler, a lowpass filter is applied
to signals before they reach the sampler.
The NI 4472 includes a two pole anti-alias lowpass filter for each input
channel. This filter has a cutoff frequency of about 400 kHz. Because its
cutoff frequency is significantly higher than the data sample rate, the analog
filter 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. In the NI 4472, the analog
sampler operates at 64 times the selected sample rate for rates above
51.2 kS/s, and at 128 times the selected sample rate for rates at and below
51.2 kS/s. For example, if you select a sample rate of 102.4 kS/s, the ADC
operates at 6.5536 MS/s (64
×
102.4 kS/s).
The analog sampler is a 1-bit ADC. The 1-bit 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
0.4863 times the sample rate) is extremely sharp, and the rejection above
0.5465 times the sample rate is greater than 110 dB. The output stage of the
digital filter resamples the higher frequency data stream at the output data
rate, producing 24-bit digital samples.
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 64 times the sample rate (for sample
rates above 51.2 kS/s) or 128 times the sample rate (for sample rates at or
below 51.2 kS/s). The analog filter in each channel rejects possible aliases
(mostly noise) from signals that lie near these multiples. Figures 3-4
and 3-5 show the frequency response of the NI 4472 input circuitry.