Chapter 3
Device Overview and Theory of Operation
3-10
ni.com
The resulting output of the filter is a band-limited signal with a dynamic
range of 102.4 kS/s down to 1.0 kS/s in 190.7 µS/s increments for
fs > 51.2 kS/s or 95.37 µS/s increments for fs £ 51.2 kS/s. 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.
Noise
The NI 4472 analog inputs typically have a dynamic range of more than
102.4 kS/s down to 1.0 kS/s in 190.7 µS/s increments for
f
s
> 51.2 kS/s
or 95.37 µS/s increments for
f
s
≤
51.2 kS/s. 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. In a 24-bit system, the largest
signal is taken to be a full-scale sine wave that peaks at the codes
+8,388,607 and -8,388,608. Such a sine wave has an RMS magnitude
of 8,388,608/1.414 = 5,932,537.482 least significant bits (LSBs).
Several factors can degrade the noise performance of the inputs. One of
these factors is noise picked up from nearby electronic devices. The
NI 4472 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
section
of Chapter 2,
, for more information.
One way to reduce the effects of noise on your measurements is to choose
the sample rate carefully. Take advantage of the anti-alias 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 anti-alias
filters reject the monitor noise outside the frequency band of interest. The
frequency response inside the band of interest is not influenced if the
sample rate is between roughly 21.6 and 28 kS/s.