LTC1966 Sallen-Key post-filtering application question
That resistor will be unfamiliar to Sallen-Key designers, since it's usually zero, making the op amp a unity gain buffer.  While it's generally useful to match impedances on the two inputs, theoretically matching the effects of input leakage current, it won't make too much difference in most designs since in a unity gain buffer the small offset will result in a similarly small offset on the output.  Presumably, two of the op amps they suggested have a higher input bias current than the other two, and this input impedance matching becomes a larger issue.