This circuit is designed by Bruce Carter from Texas Instruments. A biquad filter is a type of linear filter that implements a transfer function that is the ratio of two quadratic functions. It is available in low pass, high pass, bandpass, and notch versions. The highpass and notch versions are not recommended, because they require 4 opamps. Biquad filters are typically active and implemented with a single-amplifier biquad (SAB) or two-integrator-loop topology.
This schematic shows a typical Differential BiQuad Filter Circuit Diagram. The single-ended implementation of this filter topology has an additional op-amp to invert the output of the first op-amp. That inversion is inherent in the fully-differential op-amp, and therefore is taken directly off the first stage. This reduces the total number of op-amps required to 2.