A photoplethysmograph device comprising: a light source (102, 302, 304, 602, 702) for illuminating a target object (103, 305, 503, 603); a modulator (101, 301, 303, 601) for driving the light source such that the output intensity varies as a function of a modulation signal at a modulation frequency; a detector (105, 307, 502, 605, 703) for receiving light from the target object and generating an electrical output as a function of the intensity of received light; a bandpass filter (106, 308, 606) for filtering the detector electrical output; an analogue-to-digital converter for converting the bandpass filter output to a digital signal at a sample frequency; a demodulator (107, 309, 310, 608) for receiving the detector output, having a local oscillator and for producing 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; and means for generating, from the demodulated output, a signal indicative of blood volume as a function of time and / or blood composition; wherein the demodulator is a digital signal processing demodulator (107, 309, 310, 608), wherein further: the local oscillator is configured to produce a first square wave output at an oscillator frequency and at a first phase angle, and a 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; the demodulator is configured to multiply the detector output with the first square wave output and the second square wave output to produce an I signal and Q signal respectively; and wherein the demodulator (107, 309, 310, 608) is configured to low-pass filter and decimate the I signal and Q signal by summing samples in blocks, wherein the number of samples in a block equals the sample frequency divided by the oscillator frequency, to produce a low-pass filtered and decimated I' signal and Q' signal, the I' signal and Q' signal being de-multiplexed together to form the demodulated output. a light source (102, 302, 304, 602, 702) for illuminating a target object (103, 305, 503, 603); a modulator (101, 301, 303, 601) for driving the light source such that the output intensity varies as a function of a modulation signal at a modulation frequency; a detector (105, 307, 502, 605, 703) for receiving light from the target object and generating an electrical output as a function of the intensity of received light; a bandpass filter (106, 308, 606) for filtering the detector electrical output; an analogue-to-digital converter for converting the bandpass filter output to a digital signal at a sample frequency; a demodulator (107, 309, 310, 608) for receiving the detector output, having a local oscillator and for producing 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; and means for generating, from the demodulated output, a signal indicative of blood volume as a function of time and / or blood composition; wherein the demodulator is a digital signal processing demodulator (107, 309, 310, 608), wherein further: the local oscillator is configured to produce a first square wave output at an oscillator frequency and at a first phase angle, and a 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; the demodulator is configured to multiply the detector output with the first square wave output and the second square wave output to produce an I signal and Q signal respectively; and wherein the demodulator (107, 309, 310, 608) is configured to low-pass filter and decimate the I signal and Q signal by summing samples in blocks, wherein the number of samples in a block equals the sample frequency divided by the oscillator frequency, to produce a low-pass filtered and decimated I' signal and Q' signal, the I' signal and Q' signal being de-multiplexed together to form the demodulated output. the local oscillator is configured to produce a first square wave output at an oscillator frequency and at a first phase angle, and a 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; the demodulator is configured to multiply the detector output with the first square wave output and the second square wave output to produce an I signal and Q signal respectively; and wherein the demodulator (107, 309, 310, 608) is configured to low-pass filter and decimate the I signal and Q signal by summing samples in blocks, wherein the number of samples in a block equals the sample frequency divided by the oscillator frequency, to produce a low-pass filtered and decimated I' signal and Q' signal, the I' signal and Q' signal being de-multiplexed together to form the demodulated output.