Chapter 3
Device Overview and Theory of Operation
©
National Instruments Corporation
3-9
Figure 3-7.
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 the step response of those same
delta-sigma antialiasing filters might cause the digitized data to be clipped.
The NI 447
X
includes digital overload detection circuitry. You can
programatically poll the overload detection circuitry to monitor for an
overload condition. If an overload is detected, the data acquired at that time
may be corrupt.
The ADC
The NI 447
X
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 24 bits, rejects signal components greater than 25.6 kHz (the Nyquist
frequency), and digitally resamples the data at the more conventional rate
of 51.2 kS/s.
The 1-bit, 6.5536 MS/s from the ADC carry all the information used to
produce 24-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
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a. Clipped Signal
b. Proper Signal
