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Chapter 6 Theory of Analog Operation
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National Instruments Corporation 6-7 PCI-4451/4452 User Manual
versus sample rate is illustrated in Figure 6-3. For frequencies not near
multiples of the oversample rate, the rejection is better than 85 dB.
Figure 6-3.
Alias Rejection at the Oversample Rate
There is a form of aliasing that no filter can prevent. When a waveform
exceeds the voltage range of the ADC, it is said to be clipped or overranged.
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 6-4 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 ratio is 35 dB for the clipped
waveform and 92 dB for the properly ranged waveform. Notice that
aliases of all the harmonics due to clipping appear in Figure 6-4.
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
User.book Page 7 Tuesday, April 14, 1998 10:20 AM