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Chapter 6 Theory of Analog Operation
PCI-4451/4452 User Manual 6-8
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National Instruments Corporation
Figure 6-4.
Comparison of a Clipped Signal to a Proper Signal
An overrange can occur on the analog signal as well as on the digitized
signal. Furthermore, an analog overrange can occur independently
from a digital overrange and vice-versa. For example, a piezoelectric
accelerometer may have a resonant frequency that, when stimulated, can
produce an overrange in the analog signal, but because the delta-sigma
technology of the ADC uses very sharp antialiasing filters, the overrange is
not passed into the digitized signal. Conversely, a sharp transient on the
analog input may not overrange, but due to the step response of those same
delta-sigma antialiasing filters, the digitized data may be clipped.
The ADC
The PCI-4451/4452 ADCs use a method of A/D conversion known as
delta-sigma modulation. If the data rate is 204.8 kS/s, each ADC actually
samples its input signal at 26.2144 MS/s (128 times the data rate) and
produces 1-bit samples that are applied to the digital filter. This filter then
expands the data to 16 bits, rejects signal components greater than
102.4 kHz (the Nyquist frequency), and resamples the data at the more
conventional rate of 204.8 kS/s.
Although a 1-bit quantizer introduces a large amount of quantization error
to the signal, the 1-bit, 26 MS/s from the ADC carry all the information
used to produce 16-bit samples at 204.8 kS/s. The delta-sigma ADC
achieves this conversion from high speed to high resolution by adding a
large amount of random noise to the signal so that the resulting quantization
noise, although large, is restricted to frequencies above 102.4 kHz. This
noise is not correlated with the input signal and is almost completely
rejected by the digital filter.
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a. Clipped Signal b. Proper Signal
User.book Page 8 Tuesday, April 14, 1998 10:20 AM