Noise - National Instruments NI 447 Series User Manual

Chapter 3 Device Overview and Theory of Operation

Noise

NI 447X User Manual
correlated with the input signal and is almost completely rejected by the
digital filter.
The resulting output of the filter is a band-limited signal with a dynamic
range of more than 100 dB. One of the advantages of a delta-sigma ADC is
that it uses a 1-bit DAC as an internal reference. As a result, the delta-sigma
ADC is free from the kind of differential nonlinearity (DNL) that is
inherent in most high-resolution ADCs. This lack of DNL is especially
beneficial when the ADC is converting low-level signals, in which noise
and distortion are directly affected by converter DNL.
The NI 447X analog inputs typically have a dynamic range of more than
100 dB. The dynamic range of a circuit is the ratio of the magnitudes of the
largest signal the circuit can carry to the residual noise in the absence of a
signal.
Several factors can degrade the noise performance of the inputs. One of
these factors is noise picked up from nearby electronic devices. The
NI 447X works best when it is kept as far away as possible from other
plug-in devices, power supplies, disk drives, and computer monitors.
Cabling is also critical. Make sure to use well-shielded coaxial or balanced
cables for all connections, and route the cables away from sources of
interference such as computer monitors, switching power supplies,
and fluorescent lights. Refer to the
of Chapter 2, Using the NI
One way to reduce the effects of noise on your measurements is to choose
the sample rate carefully. Take advantage of the antialias filtering that
removes signals beyond the band of interest. Computer monitor noise,
for example, typically occurs at frequencies between 15 and 50 kHz. If the
signal of interest is restricted to below 10 kHz, for example, the antialias
filters reject the monitor noise outside the frequency band of interest, and a
sampling rate of at least 21.6 kS/s guarantees that any signal components
in the 10 kHz bandwidth of interest are acquired without aliasing and
without being attenuated by the digital filter.
Field Wiring Considerations
447X, for more information.
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