Chapter 3
Device Overview and Theory of Operation
3-8
ni.com
Figure 3-6.
Alias Rejection at the Oversample Rate
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 8,388,607 (2
23
– 1)
or
−8,388,608 (
–
2
23
)
. Clipping 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 3-7 shows the spectra of 10.5 V
peak
and 10.0 V
peak
, 3.0 kHz sine
waves. 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 caused by clipping appear in Figure 3-7a.
0.00
–10.00
–20.00
–30.00
–40.00
–50.00
–60.00
–70.00
–80.00
–90.00
–100.00
1.0
10.0
100.0
200.0
128
f
s
64
f
s
6.4 MHz
1.28 MHz
128 kHz
Alias Rejection (dB)
Sample Rate (kS/s)
Oversample
Frequency
