8.2.4 Active Filter

The OPAx320-Q1 is an excellent choice for active filter applications that require a wide bandwidth, fast slew rate, low-noise, single-supply operational amplifier. Figure 43 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, roll-off is –40 dB/dec. The Butterworth response is excellent for applications requiring predictable gain characteristics, such as the antialiasing filter used in front of an ADC.

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 the following options:

1. adding an inverting amplifier
2. adding an additional second-order MFB stage
3. using a noninverting filter topology, such as the Sallen-Key

Figure 43. Second-Order Butterworth 500-kHz Low-Pass Filter

Figure 44. OPAx320-Q1 Configured as a Three-Pole, 20-kHz, Sallen-Key Filter