Heart rate monitor and accelerometer

[juanfhj]

I want to implement a heart rate monitor for sports activity based on the principle of LED-sensor sensing through soft tissue (example Optical Heart Rate Monitor : Low Cost USB DAQ). However, my experiments show that there's a huge component due to movement of the sensor while exercising, and that component is at about the same frequency (and in fact larger) than the infrared absorption signal. I think I could place an accelerometer to measure the frequency of the motion in order to reject that frequency component. What technique could I use to reject that frequency? I think that a generic low-pass filter wouldn't work because the two frequencies are close, however, the accelerometer would also supply phase information, which may be useful.

Thanks

 

[juanfhj]

There's an alternative to the accelerometer. One would have a green LED and photoreceptor, that would measure the component of the signal due to outside disturbances, and reject it. What technique could I use to reject this signal/phase?

 

[juanfhj]

I have a clue: make the gain of the ADC inversely proportional to the amplitude of the signal. That is, a more confident measurement is taken when the interference signal is lowest, and the least confident measurement when the interfering signal is high. Also, add artificial noise when the interfering signal is high, to swamp it and make it disappear from the analysis. Finally, use instead of the Fourier transform, the "most orthogonal" function to the interfering signal (which is smooth, it's not noise-like).

Does that go by any name? How do I find the orthogonal function?

Another strategy: Sample the signal only at the confident measurement area, then suppose that this yields half the frequency of the signal.

Multiplying the non-confident sampling area by (-1)^n (analogically) makes it possible to turn the non-confident sampling interval into a confident sampling interval.

I'm also interested in knowing what would the best ADC/op amp for this. I'm new to the switched capacitor, sigma-delta, or any other architecture.