Measurement Computing IOtech 640 Series - Manual (Page 46)

6-4 Analog Signals 878893 640 & 650 Series
Low-Pass Anti-Aliasing Filter
Each of the analog input channels has its own low-pass filter to provide alias protection and to allow for the removal
of undesired frequencies from the measured response.
What is Aliasing?
Aliasing is a phenomenon of sampled data systems wherein a high frequency signal is misrepresented as a low
frequency signal when the A/D converter sampling rate being used is too slow. This misrepresentation can result in
severe data corruption and incorrect FFT results. Aliasing is a well-documented data acquisition effect, and
interested users are encouraged to research detailed information that is available on-line from companies such as
Analog Devices and Texas Instruments. This text aims to not supplant those resources, but to provide most users
with sufficient knowledge to avoid most aliasing problems through proper filter and sampling rate configuration.
For a given sampling rate, F
S
, input signals of frequency up to F
S
/2 will be processed correctly. However, input
signals above F
S
/2 are subject to aliasing. For example, a sampling rate of 100 kHz can process signals up to 50
kHz without aliasing. An input signal of 90 kHz, however, will be aliased. Specifically, it will appear in the
sampled data as a signal of frequency F
S
-F
IN
, which in this case is 100 kHz-90 kHz = 10 kHz.
Aliasing, and its prevention, should be a consideration in all sampled data systems. This is especially important in
mechanical vibration measurements, because most mechanical systems exhibit a resonance apart from their
fundamental frequency. That is, there may be signal energy present that has the potential to be aliased that is
unknown to the user. And the worst part of aliasing is that its effects are indistinguishable from real input signals.
That is, in the given example, it is not apparent to the user whether the 10 kHz energy is real or an alias.
Aliasing Protection
The 640 and 650 series analyzers have 24 bit sigma/delta analog to digital converters (ADCs). These ADCs actually
sample at rates of between 13.5 million samples per second (Msps) to 27 Msps depending on the analysis frequency
chosen in the eZ software. This high sample rate eases the requirements of the anti-aliasing filter since the Nyquist
frequency is now between 6.75MHz and 13.5MHz.
The 640 and 650 series analyzers have three pole anti-aliasing filters. Refer to the specifications chapter for details,
including a response chart.
Transducers seldom have any significant energy at these high frequencies, if there is such energy, then this filter
will
ensure it is attenuated.
Transducer Electronic Data Sheet (TEDS) Support
eZ-Analyst Only
The TEDS feature provides a 640 or 650 series device with access to the calibration information stored within
TEDS-compatible sensors. The 640 or 650 can read sensor calibration information directly from a sensor; and can
then automatically scale the readings.
TEDS is further discussed in Appendix C and in separate software documentation, when applicable.