WHY DOES ONE NEED AN ANTI-ALIASING FILTER ?
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Imagine you digitize a 1020 Hz sine wave signal at 1000 samples-per-second and plot the digitized wave on the computer display. What would you see? It turns out you would see a pure 20 Hz sine wave. The reason is because the sine wave would progress approximately one entire cycle in between digitized points, as illustrated below.

The dark blue sine wave is your original 1020 Hz signal. The tiny circles are your digitized points. And the pink line is what happens when you plot the digitized points on the computer display. The plotted wave does not look like the original, and is misleading to the viewer.

Nyquist
When one digitizes, the maximum frequency that one can measure without this “error” is half the sample rate. For example, if one digitizes at 1K samples-per-second, then the maximum frequency that one can input without it appearing as a different signal would be 500Hz. This “half the sample rate” is referred to as the “Nyquist” frequency.

Aliasing
When one digitizes waveforms greater than the nyquist, they appear differently, and this is referred to as “aliasing”. Aliasing is bad since there is no way to know if what you see is the actual signal or a signal of higher frequency.

Aliasing Error
Imagine you are interested in 0Hz to 500Hz frequencies and you digitize at 1K samples-per-second. Also imagine that 3% of the energy that enters the digitizer is > 500Hz. And imagine that you are trying to measure accurate to 1/1000. The 3% would represent much more than 1/1000, and would push your actual measured accuracy to 3%. This error is referred to as “Aliasing Error”, and is often unacceptable.

Anti-Aliasing Filter (AFS)
The solution is to analog filter out frequencies that are higher than the nyquist, before they are digitized. This involves a low pass filter that passes low frequencies and attenuates higher frequencies. The cut-off frequency of the filter, which is the cross-over frequency between pass and attenuate, is set slightly lower than the nyquist frequency. When an analog low pass filter is used in this way, it is referred to as an “anti-aliasing filter”.

instruNet i500 Anti-Aliasing Filter
The instruNet i500 anti-aliasing filter provides extremely low ±0.01 dB maximum passband ripple for Fin/Fc ≤ 0.5. If one uses further oversampling, digital filtering and desampling; then passband ripple is ±0.002 dB for Fin/Fc ≤ 0.8 due to the accuracy of a 30-pole digital filter. The end user installs between one and six i500 filter daughterboards onto each i423 card, one per channel. For details, click here.

More Information
For a video that talks about aliasing, click here and here. For a Wikipedia article, click here.