Table Of Contents

Filtering

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    Last Modified: March 30, 2018

    The FieldDAQ device uses a combination of analog and digital filtering to provide an accurate representation of in-band signals and reject out-of-band signals. The filters discriminate between signals based on the frequency range, or bandwidth, of the signal. The three important bandwidths to consider are the passband, transition band, and stopband. The FieldDAQ filters are designed to provide full alias protection from any signal component of more than one-half the sampled data rate. As such, the FieldDAQ filters track the sampled data rate and the frequency range of the passband, transition band, and stopband are all proportional to the sampled data rate.

    • Passband—The signals within the passband have frequency-dependent gain or attenuation. The small amount of variation in gain with respect to frequency is called the passband flatness. The following figure shows the typical passband flatness at three different sampled data rates. As the figure indicates, the passband always extends to 0.4 times the sampled data rate before the roll-off of the transition band begins. Refer to the FD-116xx Specifications for details on the amount of variation in the passband gain and signal delay you can expect for different frequency ranges.

      Figure 1. Typical Passband Flatness
      1. Roll-off starts at 0.4 times the data rate
    • Transition Band—The transition band spans the frequency range between 0.4 times and 0.5 times the sampled data rate. The transition band scales precisely with sampled data rate. The following figure shows the transition band frequency response and the beginning of the stopband response.

      Figure 2. Transition Band and Stopband
      1. Transition band starts at 0.4 times the sampled data rate
      2. Stopband starts at 0.5 times the sampled data rate
    • Stopband—The filter significantly attenuates all signals above the stopband frequency. The primary goal of the filter is to prevent aliasing. Therefore, the stopband frequency scales precisely to equal 0.5 times the sampled data rate. The stopband rejection is the minimum amount of attenuation applied by the filter to all signals with frequencies within the stopband.

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