The OPAx365 are an excellent choice for active filter applications requiring a wide bandwidth, fast slew rate, low-noise, single-supply operational amplifier. Figure 8-5 shows a 500-kHz, second-order, low-pass filter using the multiple-feedback (MFB) topology. The components have been selected to provide a maximally-flat Butterworth response. Beyond the cutoff frequency, rolloff is −40 dB/dec. The Butterworth response is great for applications requiring predictable gain characteristics such as the antialiasing filter used ahead of an ADC.

Figure 8-5 Second-Order Butterworth, 500-kHz Low-Pass Filter

One point to observe when considering the MFB filter is that the output is inverted, relative to the input. If this inversion is not required, or not desired, a noninverting output can be achieved through one of these options:

• Add an inverting amplifier.
• Add an additional second-order MFB stage.
• Use a noninverting filter topology such as Sallen-Key.

The Sallen-Key topology is shown in Figure 8-6.

Figure 8-6 Configured as a Three-Pole, 20-kHz, Sallen-Key Filter