Importance

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Abstract

A system for providing an improved DC and low frequency signal rejection in a photoplethysmographic measurement instrument is disclosed. The system is used in a measurement instrument which includes at least two signal sources (106, 108) for transmitting light signals at least at two wavelengths through a tissue of a test subject and a detector (112) for converting light signals transmitted through the tissue into a detector output signal. The system includes a DC restoration (114) which removes DC and low frequency signal components from the detector output signal prior to amplification thereof so as to avoid saturating amplified output signal with the low frequency signal component. The DC restoration (114) is configured to continuously remove low frequency signal component from the detector signal during dark intervals when the signal sources are deactivated, as well as during light intervals when one of the signal sources is activated. In one embodiment, the DC restoration is embodied in the form of a DC restoration circuit (200) which comprises a transimpedance amplifier (204, 206) which receives the detector output signal and produces an amplifier output signal (222) and an integrator feedback loop (208-220) which receives the amplified output signal and produces a bias current, wherein the bias current is used to subtract DC and low frequency signal components from the detector output signal prior to amplification of the detector signal by the amplifier.

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First Claim