Anti Alias Filter

In order to convert an analog signal to digital form, the signal must be sampled and quantized at regular intervals of period T. These regular intervals have a corresponding frequency of 1/T called the sampling frequency. Nyquist has established that the analog signal can be restored from these samples, without distortion, if the sampling rate is at least twice the bandwidth of the original signal. If the signal bandwidth is too great, there will be distortion. This kind of distortion is referred to as aliasing. Therefore, for a given sampling rate, the analog signal must be confined to a limited bandwidth. A lowpass filter is typically used to attenuate unwanted information in the analog signal greater than one half of the Nyquist frequency. Thus, this application of the lowpass filter is called an anti-aliasing filter because it prevents distortion due to aliasing.

In most applications the lowpass filter or anti-alias filter is placed on the input channel of the A/D converter. Lowpass filtering must be done before the signal is sampled or multiplexed, since there is no way to retrieve the original signal once it has been digitized and aliasing has occurred. In addition, after the digital signal processing has taken place, a lowpass filter is usually employed in order to clean-up harmonics created in the D/A conversion.

One advantage of a low-pass anti-alias filter is that it can reduce system cost, acquisition storage requirements, and analysis time by allowing for a lower sampling rate. Finally, a lowpass filter serves as an important element of any data acquisition system in which the accuracy of the acquired data is essential.

Under ideal conditions, a lowpass filter would pass all frequencies from DC to the filter cut-off frequency without loss or delay distortion. Frequencies above the cut-off would be eliminated, reducing the signal disturbance. But real filters gradually eliminate frequency components and thus exhibit a finite roll-off slope. In addition, group delay varies with frequency over both the passband and stopband. Attenuation slopes can have shape factors from 1.04 to 10 with ultimate attenuation from 40dB to 100dB. Filtronetics, Inc. can provide almost any anti-alias lowpass filter that meets your cut-off, delay, and attenuation requirements. These filters can be group delay and amplitude equalized, phased and amplitude matched as necessary.