Chapter 6
Theory of Analog Operation
PCI-4451/4452/4453/4454 User Manual
6-10
www.ni.com
The resulting output of the filter is a band-limited signal with a dynamic
range of over 90 dB. One of the advantages of a delta-sigma ADC is that it
uses a 1-bit DAC as an internal reference, whereas most 16-bit ADCs use
16-bit resistor-network DACs or capacitor-network DACs. 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 PCI-445X analog inputs typically have a dynamic range of more than
90 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. In a 16-bit system, the largest signal is taken to be a full-scale sine
wave that peaks at the codes +32,767 and
−
32,768. Such a sine wave has
an rms magnitude of 32,768 / 1.414 = 23,170.475 least significant bits
(LSBs).
A grounded channel of the PCI-445X has a noise level of about
0.65 LSB rms (this amount fluctuates). The ratio of 23,170.475 / 0.65 is
about 35647, or 91.0 dB—the dynamic range, but several factors can
degrade the noise performance of the inputs.
One of these factors is noise picked up from nearby electronic devices. The
PCI-445X 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 4,
, 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.
