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Chapter 6
Theory of Analog Operation
Alias Rejection (dB)
0.00
–10.00
–20.00
–30.00
–40.00
–50.00
–60.00
–70.00
–80.00
1 kS/s
Sample Rate
128 kHz
Over-Sample
Frequency
PCI-4451/4452/4453/4454 User Manual
10 kS/s
5 kS/s
640 kHz
1.28 MHz
Figure 6-4. Alias Rejection at the Oversample Rate for the PCI-4453/4454
No filter can prevent a type of aliasing caused by a clipped or overranged
waveform, that is, one that exceeds the voltage range of the ADC. When
clipping occurs, the ADC assumes the closest value in its digital range to
the actual value of the signal, which is always either −32,768 or +32,767.
Clipping nearly always results in an abrupt change in the slope of the signal
and causes the corrupted digital data to have high-frequency energy. This
energy is spread throughout the frequency spectrum, and because the
clipping happens after the antialiasing filters, the energy is aliased back into
the baseband. The remedy for this problem is simple: do not allow the
signal to exceed the nominal input range. Figure 6-5 shows the spectra of
10.5 V
and 10.0 V
rms
rms
signal-to-THD-plus-noise (THD+N) ratio is 35 dB for the clipped
waveform and 92 dB for the properly ranged waveform. Aliases of all the
harmonics due to clipping appear in Figure 6-5a.
100 kS/s
12.8 MHz
, 3.0 kHz sine waves digitized at 48 kS/s. The
6-8
1 MS/s
128 MHz
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