Patent ID: 8768424

Claim:
A photoplethysmograph device comprising: a first light source for illuminating a target object; a modulator for driving the first light source such that an output intensity varies as a function of a modulation signal at a modulation frequency; a detector for receiving light from the target object and generating an electrical output as a function of intensity of the received light; and a demodulator for receiving the detector output, having at least one local oscillator and a demodulated output representative of the modulation signal and any sidebands thereof, in which the demodulator is insensitive to any phase difference between the modulation signal and the local oscillator, the demodulator is configured to: multiply the detector output with a first and second square wave output from the at least one local oscillator to produce an I signal and Q signal respectively; and to filter and decimate the I signal and Q signal by summing samples in blocks corresponding with a cycle of an oscillator frequency to produce a filtered and decimated I′ signal and Q′ signal, and to form the demodulated output by de-multiplexing the I′ signal and Q′ signal together, wherein the demodulated output is a signal indicative of blood volume as a function of time and/or blood composition, wherein the demodulator is a digital signal processing demodulator, and the at least one local oscillator is operable to produce the first square wave output at the oscillator frequency and at a first phase angle, and the second square wave output at the oscillator frequency and at a second phase angle, the second phase angle being at 90 degrees to the first phase angle.