Chapter 6
Theory of Analog Operation
PCI-4451/4452/4453/4454 User Manual
6-8
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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
rms
and 10.0 V
rms
, 3.0 kHz sine waves digitized at 48 kS/s. The
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.
Alias Rejection (dB)
Over-Sample
Frequency
0.00
–10.00
–20.00
–30.00
–40.00
–50.00
–60.00
–70.00
–80.00
1 kS/s
10 kS/s
100 kS/s
1 MS/s
Sample Rate
128 kHz
1.28 MHz
12.8 MHz
128 MHz
5 kS/s
640 kHz
