Chapter 3
Device Overview and Theory of Operation
3-4
ni.com
f
s
≤
51.2 kS/s. This flexibility in sample rates makes the device well-suited
for a wide variety of applications, including audio and vibration analysis.
The unbalanced differential analog inputs have software-selectable AC/DC
coupling.
Calibration
The NI 4472 analog inputs have calibration adjustments. Onboard
calibration circuits remove the offset and gain errors for each channel.
For complete calibration instructions, refer to Chapter 4,
Antialias Filtering
A sampling system (such as an ADC) can represent signals of only limited
bandwidth. Specifically, a sampling rate of
f
s
can only represent signals
with a maximum frequency of
f
s
/2. This maximum frequency is known as
the
Nyquist frequency
. The bandwidth from 0 Hz to the Nyquist frequency
is the
Nyquist bandwidth
. If a signal is input to the sampling system with
frequency components that exceed the Nyquist frequency, the sampler
cannot distinguish these parts of the signal from some signals with
frequency components less than the Nyquist frequency.
For example, suppose a sampler (such as an ADC) is sampling at 1,000 S/s.
If a 400 Hz sine wave is input, then the resulting samples accurately
represent a 400 Hz sine wave. However, if a 600 Hz sine wave is input, the
resulting samples again appear to represent a 400 Hz sine wave because this
signal exceeds the Nyquist frequency (500 Hz) by 100 Hz. In fact, any sine
wave with a frequency greater than 500 Hz that is input is represented
incorrectly as a signal between 0 and 500 Hz. The apparent frequency of
this sine wave is the absolute value of the difference between the frequency
of the input signal and the closest integer multiple of 1,000 Hz (the
sampling rate). Therefore, if a 2,325 Hz sine wave is input, its apparent
frequency is as follows:
2,325
−
(2)(1,000) = 325 Hz
If a 3,975 Hz sine wave is input, its apparent frequency is as follows:
(4)(1,000)
−
3,975 = 25 Hz
The process by which the sampler modulates these higher frequency
signals back into the 0 to 500 Hz baseband is called
aliasing
.