Chapter 6
Theory of Analog Operation
©
National Instruments Corporation
6-9
PCI-4451/4452/4453/4454 User Manual
Figure 6-5.
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 might 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 might not overrange, but due to the step response of those
same delta-sigma antialiasing filters, the digitized data might be clipped.
The ADC
The PCI-445X ADC uses a method of A/D conversion known as
delta-sigma modulation. If the data rate is 51.2 kS/s, each ADC actually
samples its input signal at 6.5536 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 25.6 kHz
(the Nyquist frequency), and resamples the data at the more conventional
rate of 51.2 kS/s.
Although a 1-bit quantizer introduces a large amount of quantization error
to the signal, the 1-bit, 6.5536 MS/s from the ADC carry all the information
used to produce 16-bit samples at 51.2 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 25.6 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
