8.2.1 Active Filters

The OPAx837 is a good choice for active filters. Figure 83 and Figure 84 show MFB and Sallen-Key circuits designed implementing second-order, low-pass Butterworth filter circuits. Figure 85 illustrates the frequency response.

The main difference is that the MFB active filter provides an inverting amplifier in the pass band and the Sallen-Key active filter is noninverting. The primary advantage for each active filter is that the Sallen-Key filter in unity gain has no resistor gain error term or feedback resistor noise contribution. The MFB active filter has better attenuation properties beyond the bandwidth of the op amp. The example circuits are assuming a split-supply operation but single-supply operation is possible with midscale biasing.

Figure 83. MFB Active Filter, 100-kHz, Second-Order, Low-Pass Butterworth Filter Circuit

Figure 84. Sallen-Key Active Filter, 100-kHz, Second-Order, Low-Pass Butterworth Filter Circuit