Chapter 3
Device Overview and Theory of Operation
3-4
ni.com
The unbalanced differential analog inputs have software-selectable AC/DC
coupling.
Calibration
The NI 447
X
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
Any sampling system (such as an ADC) is limited in the bandwidth of the
signals it can represent. 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
.
A digitizer may experience input signals containing frequency components
above the Nyquist limit. It is important to understand how the digitizing
system handles these out-of-band frequencies. The NI 447
X
products
feature alias protection to eliminate these frequency components.
Many digitizers, including the successive approximation register (SAR)
ADCs often used in DAQ products, do not have alias protection. Consider
an ADC running at a sampling rate of 1,000 S/s. In this case, the Nyquist
frequency is 500 Hz. Assume the analog input signal has a frequency of
400 Hz. In this case, the ADC will accurately report the frequency of this
signal since it lies within the Nyquist bandwidth. Now assume the analog
input frequency is increased to 600 Hz; it is now 100 Hz beyond the limit
of the Nyquist bandwidth. The ADC is incapable of reproducing this
frequency digitally. The digitized data does imply a well-defined
frequency, but this frequency is inaccurate. The 600 Hz analog signal is
represented incorrectly as a 400 Hz digital waveform! In general, the
apparent frequency of a component will be the absolute value of the
difference between the actual frequency of the input signal and the closest
integer multiple of the sampling rate. If a 2,325 Hz sine wave were input to
a SAR ADC running at 1,000 S/s, its apparent frequency will be:
2,325
−
(2)(1,000) = 325 Hz
If a 3,975 Hz sine wave is input, its digital frequency is represented as
follows:
(4)(1,000)
−
3,975 = 25 Hz
