Simple and Fast Response Settling Low-Pass Filter. By introducing an extra transmission zero to the stopband of a low-pass filter, a sharp roll-off characteristic can be obtained. The filter design example of Fig. 30-l(a) shows that the time-domain performance of the low-pass section can also be improved. Figure 30-1 (b) shows the attenuation characteristic of the proposed circuit. Position of the transmission zero is determined by the passive components around the first op amp.
It was chosen to obtain 60 dB of rejection at 60 Hz. A suitable fourth-order Bessel filter has the frequency response, as shown by the dashed line. Its response to a step input is characterized by settling time to 0.1 % of 1.8 -f Fc = 180 ms. Figure 30-l(c) and 30-l(d) represent the step response for the filter of Fig. 30-l(a) in both normal and expanded voltage scales. As you can see, settling time to 0.1% is below 100 ms; overshoot and ringing, stay below 0.03%.
This quite significant speed and accuracy improvement can be a major factor, particularly for low-frequency applications. Averaging filter for low-frequency linear or true rms ac-to-dc converters is an example. Some anti-aliasing applications can also be considered. For best results, resistance ratios R4-rR5 = 20, Re + R$=1A, and capacitance ratios C3 + C2 = C3 -f C4 = 4.7 should be kept up for any selected Fc.
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