A 20-kHz, Third-Order Low-Pass Filter

This is a third order low pass filter that has corner frequency of 20kHz. It is twice as high as the auto-zero clock frequency. Here is the circuit :

The input signal operation is allowed to across its entire gain bandwidth because intermodulation  and aliasing noise are highly attenuated. This circuit also has rail-to-rail drive capability provided by amplifier’s output, allowing for a high signal-to-noise ratio at low supply voltages. [Source: Texas Instruments Application Note]