Primary source: www.nxp.com.cn/docs/en/application-note/AN4327.pdf
Verified locator: Freescale Semiconductor application note AN4327, Rev. 2, November 2012, by Santiago Lopez (cover). Sections 2.2 and 3.1.2 describe the red/infrared channels, AC/DC separation and multiplexing. The introductory wording conflates saturation with dissolved oxygen; use the note for its described optical implementation, not that physiological wording.