The Adc - National Instruments NI 4472 User Manual

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a. Clipped Signal

The ADC

© National Instruments Corporation
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 due to the step response of those
same delta-sigma antialiasing filters, the digitized data might be clipped.
The NI 4472 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 re-samples 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 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
correlated with the input signal and is almost completely rejected by the
digital filter.
Chapter 3 Device Overview and Theory of Operation
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b. Proper Signal
3-9 NI 4472 User Manual