SCM and Alias Filters
As a system component, we normally require a Signal Conditioning Module (SCM) to provide a multiple set of functions such as;
(a) isolation to prevent ground loop and associated ground noise (b) signal amplification (c) sensor linearization (d) isolated current or voltage excitation to field sensors (e) noise filtering or band pass frequency selection (f) cold junction temperature compensation and (g) simplified wiring and maintenance.

In general, most SCM have built-in Low Pass (LP) filters as part of their design. In addition to the obvious benefits of such filtering, there is the very important subtle benefit. Namely, SCM with high quality multi-pole LP filters can be viewed inherently as having anti-alias filtering capabilities. Here is how.

Signal Conditioning Module outputs typically provide inputs to multiplexing Analog to Digital Converter (ADC) units located inside either Programmable Logic Controllers (PLC) or a Data Acquisition Systems (DAS) or a computer system. These ADC units take the SCM’s analog output voltage and convert it to digital information (strings of ones and zeros) that computer computational software can manipulate. Typically this conversion is done with classic sampling techniques. This is where SCMs can be viewed as possessing the aspects of an anti-alias filter.

In any sampling situation it goes without saying that one must sample faster than the highest input signal frequency. Obviously sampling a signal that changes every second at a sample rate of once every hour would not provide adequate real-time signal information. The sampling requirement necessary to adequately represent the input signal information is to sample at a rate that is least twice the highest frequency expected in the signal. Analytical analysis of a sampling system shows that when a continuous signal is sampled at some sample frequency (fs), the resulting sampled data has a spectrum of frequencies equal to the input signal frequencies plus each input frequency added to and subtracted from each harmonic of the sampling frequency (fs).

For example, consider using a SCM with a low pass (LP) 20kHz bandwidth (3dB frequency) 2-pole filter ( attenuation slope by a factor of 100 per frequency decade, 40dB per decade) to measure a 12kHz signal containing an unexpected 60kHz noise component.. The module output feeds the input to a computer or remote data acquisition system (DAS) system using classic ADC sampling methods sampling at 100kHz. The frequency spectrum of the sampled data can contain undesirable frequency components if the 60kHz noise is not adequately attenuated prior to entering the sampling system. In this case if 60kHZ noise components are not adequately attenuated by this simple 2-pole filter, the sampled frequency spectrum would look like; 12kHz, 60kHz, 112kHz, 88kHz, 160kHz, 40kHz, 212kHz, 188kHz, 260kHz, 140kHz etc, etc. Now suppose the sampling ADC system uses the classic “reconstruction” technique of a LP filter with a cut-off frequency of half the sampling frequency (50kHz) technique for “reconstructing” the signal information. Look what happens. The reconstructed information appears to have 12kHZ and 40kHz components. This is the “Aliasing” phenomena.

Dataforth design engineers are highly skilled in filter design and use 5, 6, and 7-pole filters in Dataforth’s signal conditioning products. In the above example, the LP 2-pole filter has an attenuation slope factor of 100 per frequency decade whereas a Dataforth SCM using a 7-pole LP filter has an attenuation slope factor of 10,000,000 per frequency decade, 140dB per decade. Clearly, Dataforth SCMs provide superior filtering and essentially eliminates any “aliasing” created in a data sampling acquisition system.

Here's a typical SCM5B data sheet which describes the filters in this design...
http://www.dataforth.com/catalog/pdf/scm5b47.pdf

Remember, our Application Engineers can assist you with signal conditioner selection over the phone or via fax and email. Call us at our manufacturing facility in Tucson at 520-741-1404 (fax 520-741-0762) or Email us at techinfo@dataforth.com.
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