Ambient light auto-zero circuit for photo-optical physiological monitoring equipment

摘要

A power reduction circuit adapted to be used to measure one or more physiological properties of a patient at preset intervals by illuminating a portion of the patient with one or more light sources, which produces a reflected or transmitted modified light signal representative of the physiological property being measured; the modified light signal is detected by a photo-sensor which produces a raw output signal, which includes an ambient light component, that is representative of the modified light signal, characterised in that the power reduction circuit includes: (i) an adding unit adapted to produce a processed signal by adding the raw output signal to a second signal; (ii) a first signal buffer adapted to retain the processed signal; (iii) a processing unit which further processes the processed signal; (iv) an isolating switch which is controlled by the processing unit and is connected to the first signal buffer, said isolating switch being adapted to reversibly connect the first signal buffer to a feed back circuit; (v) said feedback circuit is adapted to generate the second signal from the processed signal, and includes a second signal buffer, which is adapted to retain the level of the processed signal inside the feedback circuit, and an inverting amplifier, which amplifies and inverts the processed signal in the second signal buffer generating the second signal; wherein the processed signal generated when the isolating switch is open, during the measurement of the desired physiological property, is effectively free of the ambient light component.