Jones H. T. ( 1993): Generate absolute value of difference. Electronic Design 24 June, 86.

Mancini R. ( 1998): Window comparator features independent adjustments. Electronic Design 9 February, 126.

Philbrick/Nexus ( 1968): Applications Manual for Operational Amplifiers, Philbrick/Nexus Research. See Section II.42. (Yes, I am one of the lucky ones to have a copy of this manual.)

Tektronix: This circuit appeared in a Tektronix magazine but they have not been able to identify it for me as I only have the single page.