Chapter 2
Using the Module
2-26
ni.com
Software Scaling and Equations
After you have acquired the signal of interest, you can scale this measurement to the
appropriate units in software. This is done automatically for you in NI-DAQmx using a strain
task or strain channel. You also can scale the measurements manually in your application
using the measurement-to-strain conversion equations provided in this document for each
configuration type. The NI PXIe-4330/4331 also supports measurements for force, pressure,
torque, bridge (V/V), and custom voltage with excitation.
Finally, there are voltage-to-strain conversion functions included in LabVIEW and
NI-DAQmx. In LabVIEW, the conversion function, Convert Strain Gage Reading VI, is in the
Data Acquisition»Signal Conditioning
subpalette. The prototypes for the NI-DAQ
functions,
Strain_Convert
and
Strain_Buf_Convert
, are in the header file
convert.h
for C/C++, and
convert.bas
for Visual Basic.
Refer to the
LabVIEW Measurements Manual
for more information. The names given the
strain-gage types in these sections directly correspond to bridge selections in MAX and the
LabVIEW Convert Strain Gage Reading VI.
Nyquist Frequency and Nyquist Bandwidth
Any sampling system, such as an ADC, is limited in the bandwidth of the signals it can
measure. Specifically, a sampling rate of
f
s
can represent only signals with frequencies lower
than
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
.
ADC
The NI PXIe-4330/4331 ADCs use a conversion method known as delta-sigma modulation.
This approach involves oversampling the input signal at a higher rate and then decimating and
filtering the resulting data to achieve the desired sample rate. For example, if the desired data
rate is 100 kS/s, each ADC actually samples its input signal at 6.4 MS/s, 64 times the data
rate, producing 1-bit samples that are sent to a digital filter. This filter rejects signal
components greater than the Nyquist frequency of 50 kHz. The 1-bit, 6.4 MS/s data stream
from the ADC contains all of the information necessary to produce 24-bit samples at 100 kS/s.
The delta-sigma ADC achieves this conversion from high speed to high resolution with a
technique called noise shaping. The ADC adds random noise to the signal so that the resulting
quantization noise, although large, is restricted to frequencies above the Nyquist frequency,
which is 50 kHz in this case. This noise is not 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 large dynamic range. 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 differential nonlinearity (DNL) and associated
noise inherent in high-resolution ADCs using other conversion techniques.
